diff --git a/.changeset/lazy-gorillas-tell.md b/.changeset/lazy-gorillas-tell.md
new file mode 100644
index 0000000000..672d8c221b
--- /dev/null
+++ b/.changeset/lazy-gorillas-tell.md
@@ -0,0 +1,7 @@
+---
+'@backstage/core-plugin-api': minor
+---
+
+Removed previously deprecated exports: `PluginHooks`, `PluginOutput`, and `FeatureFlagOutput`.
+
+The deprecated `register` method of `PluginConfig` has been removed, as well as the deprecated `output` method of `BackstagePlugin`.
diff --git a/.changeset/rotten-files-wink.md b/.changeset/rotten-files-wink.md
new file mode 100644
index 0000000000..65bd92bcc5
--- /dev/null
+++ b/.changeset/rotten-files-wink.md
@@ -0,0 +1,5 @@
+---
+'@backstage/core-components': patch
+---
+
+Updated `ResponseErrorPanel` to not use the deprecated `data` property of `ResponseError`.
diff --git a/.changeset/silver-mice-bake.md b/.changeset/silver-mice-bake.md
new file mode 100644
index 0000000000..6cba2fdb0a
--- /dev/null
+++ b/.changeset/silver-mice-bake.md
@@ -0,0 +1,5 @@
+---
+'@backstage/core-app-api': minor
+---
+
+Removed previously deprecated `ApiRegistry` export.
diff --git a/.changeset/violet-dingos-relate.md b/.changeset/violet-dingos-relate.md
new file mode 100644
index 0000000000..c8b0c03e2c
--- /dev/null
+++ b/.changeset/violet-dingos-relate.md
@@ -0,0 +1,7 @@
+---
+'@backstage/errors': minor
+---
+
+Removed the deprecated exports `ErrorResponse` and `parseErrorResponse`.
+
+Removed the deprecated `constructor` and the deprecated `data` property of `ResponseError`.
diff --git a/packages/backend-common/src/middleware/errorHandler.ts b/packages/backend-common/src/middleware/errorHandler.ts
index 47c9285b18..179d86e4c2 100644
--- a/packages/backend-common/src/middleware/errorHandler.ts
+++ b/packages/backend-common/src/middleware/errorHandler.ts
@@ -17,7 +17,7 @@
 import {
   AuthenticationError,
   ConflictError,
-  ErrorResponse,
+  ErrorResponseBody,
   InputError,
   NotAllowedError,
   NotFoundError,
@@ -89,7 +89,7 @@ export function errorHandler(
       return;
     }
 
-    const body: ErrorResponse = {
+    const body: ErrorResponseBody = {
       error: serializeError(error, { includeStack: showStackTraces }),
       request: { method: req.method, url: req.url },
       response: { statusCode },
diff --git a/packages/core-app-api/api-report.md b/packages/core-app-api/api-report.md
index f94979f975..7a6b515255 100644
--- a/packages/core-app-api/api-report.md
+++ b/packages/core-app-api/api-report.md
@@ -50,7 +50,6 @@ import { oktaAuthApiRef } from '@backstage/core-plugin-api';
 import { oneloginAuthApiRef } from '@backstage/core-plugin-api';
 import { OpenIdConnectApi } from '@backstage/core-plugin-api';
 import { PendingOAuthRequest } from '@backstage/core-plugin-api';
-import { PluginOutput } from '@backstage/core-plugin-api';
 import { ProfileInfo } from '@backstage/core-plugin-api';
 import { ProfileInfoApi } from '@backstage/core-plugin-api';
 import { PropsWithChildren } from 'react';
@@ -129,21 +128,6 @@ export type ApiProviderProps = {
   children: ReactNode;
 };
 
-// @public @deprecated
-export class ApiRegistry implements ApiHolder {
-  constructor(apis: Map<string, unknown>);
-  // Warning: (ae-forgotten-export) The symbol "ApiRegistryBuilder" needs to be exported by the entry point index.d.ts
-  //
-  // (undocumented)
-  static builder(): ApiRegistryBuilder;
-  // Warning: (ae-forgotten-export) The symbol "ApiImpl" needs to be exported by the entry point index.d.ts
-  static from(apis: ApiImpl[]): ApiRegistry;
-  // (undocumented)
-  get<T>(api: ApiRef<T>): T | undefined;
-  static with<T>(api: ApiRef<T>, impl: T): ApiRegistry;
-  with<T>(api: ApiRef<T>, impl: T): ApiRegistry;
-}
-
 // @public
 export class ApiResolver implements ApiHolder {
   constructor(factories: ApiFactoryHolder);
@@ -208,14 +192,19 @@ export type AppOptions = {
   icons: AppIcons & {
     [key in string]: IconComponent;
   };
-  plugins?: (Omit<BackstagePlugin<any, any>, 'output'> & {
-    output(): (
-      | PluginOutput
-      | {
-          type: string;
-        }
-    )[];
-  })[];
+  plugins?: Array<
+    BackstagePlugin<any, any> & {
+      output?(): Array<
+        | {
+            type: 'feature-flag';
+            name: string;
+          }
+        | {
+            type: string;
+          }
+      >;
+    }
+  >;
   components: AppComponents;
   themes: (Partial<AppTheme> & Omit<AppTheme, 'theme'>)[];
   configLoader?: AppConfigLoader;
diff --git a/packages/core-app-api/src/apis/system/ApiRegistry.ts b/packages/core-app-api/src/apis/system/ApiRegistry.ts
index e433381dd1..d4990a04c5 100644
--- a/packages/core-app-api/src/apis/system/ApiRegistry.ts
+++ b/packages/core-app-api/src/apis/system/ApiRegistry.ts
@@ -18,6 +18,7 @@ import { ApiRef, ApiHolder } from '@backstage/core-plugin-api';
 
 type ApiImpl<T = unknown> = readonly [ApiRef<T>, T];
 
+/** @internal */
 class ApiRegistryBuilder {
   private apis: [string, unknown][] = [];
 
@@ -35,8 +36,7 @@ class ApiRegistryBuilder {
 /**
  * A registry for utility APIs.
  *
- * @public
- * @deprecated Will be removed, use {@link @backstage/test-utils#TestApiProvider} or {@link @backstage/test-utils#TestApiRegistry} instead.
+ * @internal
  */
 export class ApiRegistry implements ApiHolder {
   static builder() {
diff --git a/packages/core-app-api/src/apis/system/index.ts b/packages/core-app-api/src/apis/system/index.ts
index 56c42f1e2a..3c0b715249 100644
--- a/packages/core-app-api/src/apis/system/index.ts
+++ b/packages/core-app-api/src/apis/system/index.ts
@@ -16,7 +16,6 @@
 
 export { ApiProvider } from './ApiProvider';
 export type { ApiProviderProps } from './ApiProvider';
-export { ApiRegistry } from './ApiRegistry';
 export { ApiResolver } from './ApiResolver';
 export { ApiFactoryRegistry } from './ApiFactoryRegistry';
 export type { ApiFactoryScope } from './ApiFactoryRegistry';
diff --git a/packages/core-app-api/src/app/AppManager.test.tsx b/packages/core-app-api/src/app/AppManager.test.tsx
index 20872b26f0..b09d424737 100644
--- a/packages/core-app-api/src/app/AppManager.test.tsx
+++ b/packages/core-app-api/src/app/AppManager.test.tsx
@@ -316,7 +316,7 @@ describe('Integration Test', () => {
       plugins: [
         createPlugin({
           id: 'test',
-          register: p => p.featureFlags.register('name'),
+          featureFlags: [{ name: 'name' }],
         }),
       ],
       components,
@@ -384,8 +384,24 @@ describe('Integration Test', () => {
               name: 'foo',
             },
           ],
-          register: p => p.featureFlags.register('name'),
         }),
+        // We still support consuming the old feature flag API for a little while longer
+        {
+          getId() {
+            return 'old-test';
+          },
+          getApis() {
+            return [];
+          },
+          output() {
+            return [
+              {
+                type: 'feature-flag',
+                name: 'old-feature-flag',
+              },
+            ];
+          },
+        } as any,
       ],
       components,
       configLoader: async () => [],
@@ -412,12 +428,12 @@ describe('Integration Test', () => {
     );
 
     expect(storageFlags.registerFlag).toHaveBeenCalledWith({
-      name: 'name',
+      name: 'foo',
       pluginId: 'test',
     });
     expect(storageFlags.registerFlag).toHaveBeenCalledWith({
-      name: 'foo',
-      pluginId: 'test',
+      name: 'old-feature-flag',
+      pluginId: 'old-test',
     });
   });
 
diff --git a/packages/core-app-api/src/app/AppManager.tsx b/packages/core-app-api/src/app/AppManager.tsx
index 7cdc06a094..ef5fe43b8b 100644
--- a/packages/core-app-api/src/app/AppManager.tsx
+++ b/packages/core-app-api/src/app/AppManager.tsx
@@ -27,7 +27,6 @@ import { Route, Routes } from 'react-router-dom';
 import useAsync from 'react-use/lib/useAsync';
 import {
   ApiProvider,
-  ApiRegistry,
   AppThemeSelector,
   ConfigReader,
   LocalStorageFeatureFlags,
@@ -80,6 +79,13 @@ import {
 } from './types';
 import { AppThemeProvider } from './AppThemeProvider';
 import { defaultConfigLoader } from './defaultConfigLoader';
+import { ApiRegistry } from '../apis/system/ApiRegistry';
+
+type CompatiblePlugin =
+  | BackstagePlugin<any, any>
+  | (Omit<BackstagePlugin<any, any>, 'getFeatureFlags'> & {
+      output(): Array<{ type: 'feature-flag'; name: string }>;
+    });
 
 export function generateBoundRoutes(bindRoutes: AppOptions['bindRoutes']) {
   const result = new Map<ExternalRouteRef, RouteRef | SubRouteRef>();
@@ -149,7 +155,7 @@ function useConfigLoader(
   if (noConfigNode) {
     return {
       node: (
-        <ApiProvider apis={ApiRegistry.from([[appThemeApiRef, appThemeApi]])}>
+        <ApiProvider apis={ApiRegistry.with(appThemeApiRef, appThemeApi)}>
           <ThemeProvider>{noConfigNode}</ThemeProvider>
         </ApiProvider>
       ),
@@ -183,7 +189,7 @@ export class AppManager implements BackstageApp {
 
   private readonly apis: Iterable<AnyApiFactory>;
   private readonly icons: NonNullable<AppOptions['icons']>;
-  private readonly plugins: Set<BackstagePlugin<any, any>>;
+  private readonly plugins: Set<CompatiblePlugin>;
   private readonly components: AppComponents;
   private readonly themes: AppTheme[];
   private readonly configLoader?: AppConfigLoader;
@@ -196,9 +202,7 @@ export class AppManager implements BackstageApp {
   constructor(options: AppOptions) {
     this.apis = options.apis ?? [];
     this.icons = options.icons;
-    this.plugins = new Set(
-      (options.plugins as BackstagePlugin<any, any>[]) ?? [],
-    );
+    this.plugins = new Set((options.plugins as CompatiblePlugin[]) ?? []);
     this.components = options.components;
     this.themes = options.themes as AppTheme[];
     this.configLoader = options.configLoader ?? defaultConfigLoader;
@@ -208,7 +212,7 @@ export class AppManager implements BackstageApp {
   }
 
   getPlugins(): BackstagePlugin<any, any>[] {
-    return Array.from(this.plugins);
+    return Array.from(this.plugins) as BackstagePlugin<any, any>[];
   }
 
   getSystemIcon(key: string): IconComponent | undefined {
@@ -282,16 +286,11 @@ export class AppManager implements BackstageApp {
               }
             } else {
               for (const output of plugin.output()) {
-                switch (output.type) {
-                  case 'feature-flag': {
-                    featureFlagsApi.registerFlag({
-                      name: output.name,
-                      pluginId: plugin.getId(),
-                    });
-                    break;
-                  }
-                  default:
-                    break;
+                if (output.type === 'feature-flag') {
+                  featureFlagsApi.registerFlag({
+                    name: output.name,
+                    pluginId: plugin.getId(),
+                  });
                 }
               }
             }
@@ -468,7 +467,7 @@ export class AppManager implements BackstageApp {
     return this.apiHolder;
   }
 
-  private verifyPlugins(plugins: Iterable<BackstagePlugin>) {
+  private verifyPlugins(plugins: Iterable<CompatiblePlugin>) {
     const pluginIds = new Set<string>();
 
     for (const plugin of plugins) {
diff --git a/packages/core-app-api/src/app/types.ts b/packages/core-app-api/src/app/types.ts
index 4b8d2741cb..bacaf83fbd 100644
--- a/packages/core-app-api/src/app/types.ts
+++ b/packages/core-app-api/src/app/types.ts
@@ -24,7 +24,6 @@ import {
   RouteRef,
   SubRouteRef,
   ExternalRouteRef,
-  PluginOutput,
   IdentityApi,
 } from '@backstage/core-plugin-api';
 import { AppConfig } from '@backstage/config';
@@ -232,9 +231,13 @@ export type AppOptions = {
   /**
    * A list of all plugins to include in the app.
    */
-  plugins?: (Omit<BackstagePlugin<any, any>, 'output'> & {
-    output(): (PluginOutput | { type: string })[];
-  })[];
+  plugins?: Array<
+    BackstagePlugin<any, any> & {
+      output?(): Array<
+        { type: 'feature-flag'; name: string } | { type: string }
+      >; // support for old plugins
+    }
+  >;
 
   /**
    * Supply components to the app to override the default ones.
diff --git a/packages/core-components/src/components/ResponseErrorPanel/ResponseErrorPanel.tsx b/packages/core-components/src/components/ResponseErrorPanel/ResponseErrorPanel.tsx
index 68d7360400..9600a3f179 100644
--- a/packages/core-components/src/components/ResponseErrorPanel/ResponseErrorPanel.tsx
+++ b/packages/core-components/src/components/ResponseErrorPanel/ResponseErrorPanel.tsx
@@ -61,14 +61,14 @@ export function ResponseErrorPanel(props: ErrorPanelProps) {
     );
   }
 
-  const { data, cause } = error as ResponseError;
-  const { request, response } = data;
+  const { body, cause } = error as ResponseError;
+  const { request, response } = body;
 
   const errorString = `${response.statusCode}: ${cause.name}`;
   const requestString = request && `${request.method} ${request.url}`;
   const messageString = cause.message.replace(/\\n/g, '\n');
   const stackString = cause.stack?.replace(/\\n/g, '\n');
-  const jsonString = JSON.stringify(data, undefined, 2);
+  const jsonString = JSON.stringify(body, undefined, 2);
 
   return (
     <ErrorPanel
diff --git a/packages/core-plugin-api/api-report.md b/packages/core-plugin-api/api-report.md
index 3a4c7580b2..7a2fb95be8 100644
--- a/packages/core-plugin-api/api-report.md
+++ b/packages/core-plugin-api/api-report.md
@@ -241,7 +241,6 @@ export type BackstagePlugin<
   ExternalRoutes extends AnyExternalRoutes = {},
 > = {
   getId(): string;
-  output(): PluginOutput[];
   getApis(): Iterable<AnyApiFactory>;
   getFeatureFlags(): Iterable<PluginFeatureFlagConfig>;
   provide<T>(extension: Extension<T>): T;
@@ -446,12 +445,6 @@ export type FeatureFlag = {
   pluginId: string;
 };
 
-// @public @deprecated
-export type FeatureFlagOutput = {
-  type: 'feature-flag';
-  name: string;
-};
-
 // @public
 export interface FeatureFlagsApi {
   getRegisteredFlags(): FeatureFlag[];
@@ -687,7 +680,6 @@ export type PluginConfig<
 > = {
   id: string;
   apis?: Iterable<AnyApiFactory>;
-  register?(hooks: PluginHooks): void;
   routes?: Routes;
   externalRoutes?: ExternalRoutes;
   featureFlags?: PluginFeatureFlagConfig[];
@@ -698,14 +690,6 @@ export type PluginFeatureFlagConfig = {
   name: string;
 };
 
-// @public @deprecated
-export type PluginHooks = {
-  featureFlags: FeatureFlagsHooks;
-};
-
-// @public @deprecated
-export type PluginOutput = FeatureFlagOutput;
-
 // @public
 export type ProfileInfo = {
   email?: string;
diff --git a/packages/core-plugin-api/src/plugin/Plugin.test.tsx b/packages/core-plugin-api/src/plugin/Plugin.test.tsx
index d3758543a1..b66235e210 100644
--- a/packages/core-plugin-api/src/plugin/Plugin.test.tsx
+++ b/packages/core-plugin-api/src/plugin/Plugin.test.tsx
@@ -28,62 +28,7 @@ describe('Plugin Feature Flag', () => {
     expect(
       createPlugin({
         id: 'test',
-        register({ featureFlags }) {
-          featureFlags.register('blob');
-        },
       }).getFeatureFlags(),
-    ).toEqual([{ name: 'blob' }]);
-
-    expect(
-      createPlugin({
-        id: 'test',
-        register({ featureFlags }) {
-          featureFlags.register('blob');
-        },
-        featureFlags: [{ name: 'test' }],
-      }).getFeatureFlags(),
-    ).toEqual([{ name: 'test' }, { name: 'blob' }]);
-
-    expect(
-      createPlugin({
-        id: 'test',
-      }).getFeatureFlags(),
-    ).toEqual([]);
-
-    /* deprecated tests */
-
-    expect(
-      createPlugin({
-        id: 'test',
-        featureFlags: [{ name: 'test' }],
-      }).output(),
-    ).toEqual([{ name: 'test', type: 'feature-flag' }]);
-
-    expect(
-      createPlugin({
-        id: 'test',
-        register({ featureFlags }) {
-          featureFlags.register('blob');
-        },
-      }).output(),
-    ).toEqual([{ name: 'blob', type: 'feature-flag' }]);
-    expect(
-      createPlugin({
-        id: 'test',
-        register({ featureFlags }) {
-          featureFlags.register('blob');
-        },
-        featureFlags: [{ name: 'test' }],
-      }).output(),
-    ).toEqual([
-      { name: 'test', type: 'feature-flag' },
-      { name: 'blob', type: 'feature-flag' },
-    ]);
-
-    expect(
-      createPlugin({
-        id: 'test',
-      }).output(),
     ).toEqual([]);
   });
 });
diff --git a/packages/core-plugin-api/src/plugin/Plugin.tsx b/packages/core-plugin-api/src/plugin/Plugin.tsx
index 0fb7574cd7..d97554b27b 100644
--- a/packages/core-plugin-api/src/plugin/Plugin.tsx
+++ b/packages/core-plugin-api/src/plugin/Plugin.tsx
@@ -16,7 +16,6 @@
 
 import {
   PluginConfig,
-  PluginOutput,
   BackstagePlugin,
   Extension,
   AnyRoutes,
@@ -33,8 +32,6 @@ export class PluginImpl<
   ExternalRoutes extends AnyExternalRoutes,
 > implements BackstagePlugin<Routes, ExternalRoutes>
 {
-  private storedOutput?: PluginOutput[];
-
   constructor(private readonly config: PluginConfig<Routes, ExternalRoutes>) {}
 
   getId(): string {
@@ -46,11 +43,7 @@ export class PluginImpl<
   }
 
   getFeatureFlags(): Iterable<PluginFeatureFlagConfig> {
-    const registeredFlags = this.output()
-      .filter(({ type }) => type === 'feature-flag')
-      .map(({ name }) => ({ name }));
-
-    return registeredFlags;
+    return this.config.featureFlags?.slice() ?? [];
   }
 
   get routes(): Routes {
@@ -61,34 +54,6 @@ export class PluginImpl<
     return this.config.externalRoutes ?? ({} as ExternalRoutes);
   }
 
-  output(): PluginOutput[] {
-    if (this.storedOutput) {
-      return this.storedOutput;
-    }
-    const outputs = new Array<PluginOutput>();
-    this.storedOutput = outputs;
-
-    if (this.config.featureFlags) {
-      for (const flag of this.config.featureFlags) {
-        outputs.push({ type: 'feature-flag', name: flag.name });
-      }
-    }
-
-    if (!this.config.register) {
-      return outputs;
-    }
-
-    this.config.register({
-      featureFlags: {
-        register(name) {
-          outputs.push({ type: 'feature-flag', name });
-        },
-      },
-    });
-
-    return this.storedOutput;
-  }
-
   provide<T>(extension: Extension<T>): T {
     return extension.expose(this);
   }
diff --git a/packages/core-plugin-api/src/plugin/index.ts b/packages/core-plugin-api/src/plugin/index.ts
index 4f7609fd9c..95f5a6b219 100644
--- a/packages/core-plugin-api/src/plugin/index.ts
+++ b/packages/core-plugin-api/src/plugin/index.ts
@@ -20,10 +20,7 @@ export type {
   AnyRoutes,
   BackstagePlugin,
   Extension,
-  FeatureFlagOutput,
   FeatureFlagsHooks,
   PluginConfig,
-  PluginHooks,
-  PluginOutput,
   PluginFeatureFlagConfig,
 } from './types';
diff --git a/packages/core-plugin-api/src/plugin/types.ts b/packages/core-plugin-api/src/plugin/types.ts
index 0f8d3404d8..d639327838 100644
--- a/packages/core-plugin-api/src/plugin/types.ts
+++ b/packages/core-plugin-api/src/plugin/types.ts
@@ -17,24 +17,6 @@
 import { RouteRef, SubRouteRef, ExternalRouteRef } from '../routing';
 import { AnyApiFactory } from '../apis/system';
 
-/**
- * Replace with using {@link RouteRef}s.
- * @deprecated will be removed
- * @public
- */
-export type FeatureFlagOutput = {
-  type: 'feature-flag';
-  name: string;
-};
-
-/**
- * {@link FeatureFlagOutput} type.
- *
- * @public
- * @deprecated Use {@link BackstagePlugin.getFeatureFlags} instead.
- */
-export type PluginOutput = FeatureFlagOutput;
-
 /**
  * Plugin extension type.
  *
@@ -72,10 +54,6 @@ export type BackstagePlugin<
   ExternalRoutes extends AnyExternalRoutes = {},
 > = {
   getId(): string;
-  /**
-   * @deprecated use getFeatureFlags instead.
-   * */
-  output(): PluginOutput[];
   getApis(): Iterable<AnyApiFactory>;
   /**
    * Returns all registered feature flags for this plugin.
@@ -107,23 +85,11 @@ export type PluginConfig<
 > = {
   id: string;
   apis?: Iterable<AnyApiFactory>;
-  /** @deprecated use featureFlags property instead for defining feature flags */
-  register?(hooks: PluginHooks): void;
   routes?: Routes;
   externalRoutes?: ExternalRoutes;
   featureFlags?: PluginFeatureFlagConfig[];
 };
 
-/**
- * Holds hooks registered by the plugin.
- *
- * @deprecated - feature flags are now registered in plugin config under featureFlags
- * @public
- */
-export type PluginHooks = {
-  featureFlags: FeatureFlagsHooks;
-};
-
 /**
  * Interface for registering feature flags hooks.
  *
diff --git a/packages/errors/api-report.md b/packages/errors/api-report.md
index 97b011bad6..a01581135b 100644
--- a/packages/errors/api-report.md
+++ b/packages/errors/api-report.md
@@ -33,9 +33,6 @@ export type ErrorLike = {
   [unknownKeys: string]: unknown;
 };
 
-// @public @deprecated
-export type ErrorResponse = ErrorResponseBody;
-
 // @public
 export type ErrorResponseBody = {
   error: SerializedError;
@@ -68,9 +65,6 @@ export class NotFoundError extends CustomErrorBase {}
 // @public
 export class NotModifiedError extends CustomErrorBase {}
 
-// @public @deprecated
-export function parseErrorResponse(response: Response): Promise<ErrorResponse>;
-
 // @public
 export function parseErrorResponseBody(
   response: Response,
@@ -78,17 +72,8 @@ export function parseErrorResponseBody(
 
 // @public
 export class ResponseError extends Error {
-  // @deprecated
-  constructor(props: {
-    message: string;
-    response: Response;
-    data: ErrorResponseBody;
-    cause: Error;
-  });
   readonly body: ErrorResponseBody;
   readonly cause: Error;
-  // @deprecated
-  get data(): ErrorResponseBody;
   static fromResponse(response: Response): Promise<ResponseError>;
   readonly response: Response;
 }
diff --git a/packages/errors/src/errors/ResponseError.test.ts b/packages/errors/src/errors/ResponseError.test.ts
index 52a308dbb8..94f2e850d9 100644
--- a/packages/errors/src/errors/ResponseError.test.ts
+++ b/packages/errors/src/errors/ResponseError.test.ts
@@ -14,12 +14,12 @@
  * limitations under the License.
  */
 
-import { ErrorResponse } from '../serialization';
+import { ErrorResponseBody } from '../serialization';
 import { ResponseError } from './ResponseError';
 
 describe('ResponseError', () => {
   it('constructs itself from a response', async () => {
-    const body: ErrorResponse = {
+    const body: ErrorResponseBody = {
       error: { name: 'Fours', message: 'Expected fives', stack: 'lines' },
       request: { method: 'GET', url: '/' },
       response: { statusCode: 444 },
diff --git a/packages/errors/src/errors/ResponseError.ts b/packages/errors/src/errors/ResponseError.ts
index a20035cd8c..84c8137bb5 100644
--- a/packages/errors/src/errors/ResponseError.ts
+++ b/packages/errors/src/errors/ResponseError.ts
@@ -75,10 +75,7 @@ export class ResponseError extends Error {
     });
   }
 
-  /**
-   * @deprecated will be removed.
-   **/
-  constructor(props: {
+  private constructor(props: {
     message: string;
     response: Response;
     data: ErrorResponseBody;
@@ -90,13 +87,4 @@ export class ResponseError extends Error {
     this.body = props.data;
     this.cause = props.cause;
   }
-  /**
-   * The parsed JSON error body, as sent by the server.
-   * @deprecated use body instead.
-   */
-  get data(): ErrorResponseBody {
-    // eslint-disable-next-line no-console
-    console.warn('ErrorResponse.data is deprecated, use .body instead.');
-    return this.body;
-  }
 }
diff --git a/packages/errors/src/serialization/index.ts b/packages/errors/src/serialization/index.ts
index b86661d496..b03514f749 100644
--- a/packages/errors/src/serialization/index.ts
+++ b/packages/errors/src/serialization/index.ts
@@ -16,5 +16,5 @@
 
 export { deserializeError, serializeError, stringifyError } from './error';
 export type { SerializedError } from './error';
-export { parseErrorResponse, parseErrorResponseBody } from './response';
-export type { ErrorResponse, ErrorResponseBody } from './response';
+export { parseErrorResponseBody } from './response';
+export type { ErrorResponseBody } from './response';
diff --git a/packages/errors/src/serialization/response.ts b/packages/errors/src/serialization/response.ts
index 4a1731646d..3580220525 100644
--- a/packages/errors/src/serialization/response.ts
+++ b/packages/errors/src/serialization/response.ts
@@ -16,14 +16,6 @@
 
 import { SerializedError } from './error';
 
-/**
- * A standard shape of JSON data returned as the body of backend errors.
- *
- * @public
- * @deprecated - Use {@link ErrorResponseBody} instead.
- */
-export type ErrorResponse = ErrorResponseBody;
-
 /**
  * A standard shape of JSON data returned as the body of backend errors.
  *
@@ -48,25 +40,6 @@ export type ErrorResponseBody = {
   };
 };
 
-/**
- * Attempts to construct an ErrorResponse out of a failed server request.
- * Assumes that the response has already been checked to be not ok. This
- * function consumes the body of the response, and assumes that it hasn't
- * been consumed before.
- *
- * The code is forgiving, and constructs a useful synthetic body as best it can
- * if the response body wasn't on the expected form.
- *
- * @public
- * @param response - The response of a failed request
- * @deprecated - Use {@link parseErrorResponseBody} instead.
- */
-export async function parseErrorResponse(
-  response: Response,
-): Promise<ErrorResponse> {
-  return parseErrorResponseBody(response);
-}
-
 /**
  * Attempts to construct an ErrorResponseBody out of a failed server request.
  * Assumes that the response has already been checked to be not ok. This