diff --git a/docs/frontend-system/building-plugins/01-index.md b/docs/frontend-system/building-plugins/01-index.md
index 861f454280..795249c73f 100644
--- a/docs/frontend-system/building-plugins/01-index.md
+++ b/docs/frontend-system/building-plugins/01-index.md
@@ -238,3 +238,12 @@ export const examplePlugin = createFrontendPlugin({
 The `ExampleEntityContent` itself is again a regular React component where you can implement any functionality you want. To access the entity that the content is being rendered for, you can use the `useEntity` hook from `@backstage/plugin-catalog-react`. You can see a full list of APIs provided by the catalog React library in [the API reference](https://backstage.io/api/stable/modules/_backstage_plugin-catalog-react.index.html).
 
 For a more complete list of the different kinds of extensions that you can create for your plugin, see the [extension blueprints](./03-common-extension-blueprints.md) section.
+
+## Related topics
+
+The following guides cover cross-cutting concerns that apply to both the old and new frontend systems:
+
+- [Internationalization (i18n)](../../plugins/internationalization.md) — Adding translations to your plugin using `createTranslationRef` and `useTranslationRef`.
+- [Plugin Analytics](../../plugins/analytics.md) — Instrumenting user interactions with the Analytics API, including the `AnalyticsImplementationBlueprint` for the new frontend system.
+- [Feature Flags](../../plugins/feature-flags.md) — Defining and using feature flags. In the new frontend system, flags are declared in the `featureFlags` option of `createFrontendPlugin`.
+- [Integrating Search into Plugins](../../plugins/integrating-search-into-plugins.md) — Building search experiences and collators. For the new frontend system, use `SearchResultListItemBlueprint` from `@backstage/plugin-search-react/alpha`.
diff --git a/docs/plugins/add-to-directory.md b/docs/plugins/add-to-directory.md
index 48affef489..a87062e207 100644
--- a/docs/plugins/add-to-directory.md
+++ b/docs/plugins/add-to-directory.md
@@ -4,6 +4,12 @@ title: Add to Directory
 description: Documentation on Adding Plugin to Plugin Directory
 ---
 
+:::caution Legacy Documentation
+
+This section is part of the legacy plugins documentation. The process for adding plugins to the directory described here is still current.
+
+:::
+
 ## Adding a Plugin to the Directory
 
 To add a new plugin to the [plugin directory](https://backstage.io/plugins) create a file with the following pattern `<plugin-name>.yaml` where `<plugin-name>` is the name of your plugin. This file will go in [`microsite/data/plugins`](https://github.com/backstage/backstage/tree/master/microsite/data/plugins) with your plugin's information. Example:
diff --git a/docs/plugins/analytics.md b/docs/plugins/analytics.md
index 3e8d5b4111..692849c1eb 100644
--- a/docs/plugins/analytics.md
+++ b/docs/plugins/analytics.md
@@ -4,6 +4,12 @@ title: Plugin Analytics
 description: Measuring usage of your Backstage instance.
 ---
 
+:::caution Legacy Documentation
+
+This section is part of the legacy plugins documentation. The concepts and events described here apply to both the old and new frontend systems. However, the "Writing Integrations" section shows examples for both the old frontend system (`createApiFactory`) and the new frontend system (`AnalyticsImplementationBlueprint`). For new development, prefer the new frontend system approach. See also [Utility APIs](../frontend-system/utility-apis/01-index.md).
+
+:::
+
 Setting up, maintaining, and iterating on an instance of Backstage can be a
 large investment. To help measure return on this investment, Backstage comes
 with an event-based Analytics API that grants app integrators the flexibility to
diff --git a/docs/plugins/backend-plugin.md b/docs/plugins/backend-plugin.md
index 2872fdcda9..91a3e04839 100644
--- a/docs/plugins/backend-plugin.md
+++ b/docs/plugins/backend-plugin.md
@@ -4,6 +4,12 @@ title: Backend plugins
 description: Creating and Developing Backend plugins
 ---
 
+:::caution Legacy Documentation
+
+This section is part of the legacy plugins documentation. While this page already describes the new backend system patterns, the canonical documentation for building backend plugins has moved to [Building Backend Plugins and Modules](../backend-system/building-plugins-and-modules/01-index.md).
+
+:::
+
 This page describes the process of creating and managing backend plugins in your
 Backstage repository.
 
diff --git a/docs/plugins/call-existing-api.md b/docs/plugins/call-existing-api.md
index 35a6d8d599..5b5a91b708 100644
--- a/docs/plugins/call-existing-api.md
+++ b/docs/plugins/call-existing-api.md
@@ -4,6 +4,12 @@ title: Call Existing API
 description: Describes the various options that Backstage frontend plugins have, in communicating with service APIs that already exist
 ---
 
+:::caution Legacy Documentation
+
+This section is part of the legacy plugins documentation. The frontend code examples on this page use the old frontend system APIs (`discoveryApiRef`, `fetchApiRef` from `@backstage/core-plugin-api`). The same APIs are available in the new frontend system via `@backstage/frontend-plugin-api`. The general guidance on when to use direct requests vs. the proxy vs. a backend plugin remains valid for both systems.
+
+:::
+
 This article describes the various options that Backstage frontend plugins have,
 in communicating with service APIs that already exist. Each section below
 describes a possible choice, and the circumstances under which it fits.
diff --git a/docs/plugins/composability.md b/docs/plugins/composability.md
index d6c9abc98f..35da80a5b2 100644
--- a/docs/plugins/composability.md
+++ b/docs/plugins/composability.md
@@ -4,6 +4,12 @@ title: Composability System
 description: Documentation for the Backstage plugin composability APIs.
 ---
 
+:::caution Legacy Documentation
+
+This page describes the composability system for the **old frontend system**, including `createRoutableExtension`, `createComponentExtension`, `RouteRef`, `ExternalRouteRef`, and component data. For the new frontend system, see [Extensions](../frontend-system/architecture/20-extensions.md), [Extension Blueprints](../frontend-system/architecture/23-extension-blueprints.md), and [Routes](../frontend-system/architecture/36-routes.md).
+
+:::
+
 ## Summary
 
 This page describes the composability system that helps bring together content
diff --git a/docs/plugins/create-a-plugin.md b/docs/plugins/create-a-plugin.md
index ea0b088af7..8c97a3d30d 100644
--- a/docs/plugins/create-a-plugin.md
+++ b/docs/plugins/create-a-plugin.md
@@ -4,6 +4,12 @@ title: Create a Backstage Plugin
 description: Documentation on How to Create a Backstage Plugin
 ---
 
+:::caution Legacy Documentation
+
+This page describes creating plugins for the **old frontend system**. For creating plugins using the new frontend system, see [Building Frontend Plugins](../frontend-system/building-plugins/01-index.md). For creating backend plugins, see [Building Backend Plugins and Modules](../backend-system/building-plugins-and-modules/01-index.md).
+
+:::
+
 A Backstage Plugin adds functionality to Backstage.
 
 ## Create a Plugin
diff --git a/docs/plugins/feature-flags.md b/docs/plugins/feature-flags.md
index 2d0e34ba4b..5d35e3c057 100644
--- a/docs/plugins/feature-flags.md
+++ b/docs/plugins/feature-flags.md
@@ -4,6 +4,12 @@ title: Feature Flags
 description: Details the process of defining setting and reading a feature flag.
 ---
 
+:::caution Legacy Documentation
+
+This page describes feature flags using the **old frontend system** APIs (`createPlugin` from `@backstage/core-plugin-api` and `createApp` from `@backstage/app-defaults`). For the new frontend system, feature flags are declared via the `featureFlags` option in `createFrontendPlugin` — see [Building Frontend Plugins](../frontend-system/building-plugins/01-index.md). The `FeatureFlagged` component and `featureFlagsApiRef` work the same way in both systems.
+
+:::
+
 Backstage offers the ability to define feature flags inside a plugin or during application creation. This allows you to restrict parts of your plugin to those individual users who have toggled the feature flag to on.
 
 This page describes the process of defining, setting and reading a feature flag. If you are looking for using feature flags specifically with software templates please see [Writing Templates](https://backstage.io/docs/features/software-templates/writing-templates#remove-sections-or-fields-based-on-feature-flags).
diff --git a/docs/plugins/index.md b/docs/plugins/index.md
index ef45152f55..388c3b1177 100644
--- a/docs/plugins/index.md
+++ b/docs/plugins/index.md
@@ -1,9 +1,15 @@
 ---
 id: index
-title: Introduction to Plugins
-description: Learn about integrating various infrastructure and software development tools into Backstage through plugins.
+title: Introduction to Plugins (Legacy)
+description: Legacy documentation for integrating various infrastructure and software development tools into Backstage through plugins using the old frontend system.
 ---
 
+:::caution Legacy Documentation
+
+This section covers plugin development using the **old frontend system**. For new development, please refer to the [new frontend system](../frontend-system/index.md) and [new backend system](../backend-system/index.md) documentation. The content here is kept for reference and for maintaining existing plugins that have not yet been migrated.
+
+:::
+
 Backstage orchestrates a cohesive single-page application by seamlessly integrating various plugins.
 
 Our vision for the plugin ecosystem champions flexibility, empowering you to incorporate a broad spectrum of infrastructure and software development tools into Backstage as plugins. Adherence to stringent [design guidelines](../dls/design.md) guarantees a consistent and intuitive user experience across the entire plugin landscape.
diff --git a/docs/plugins/integrating-plugin-into-software-catalog.md b/docs/plugins/integrating-plugin-into-software-catalog.md
index 417b932451..1e3ebb2d2b 100644
--- a/docs/plugins/integrating-plugin-into-software-catalog.md
+++ b/docs/plugins/integrating-plugin-into-software-catalog.md
@@ -4,6 +4,12 @@ title: Integrate into the Software Catalog
 description: How to integrate a plugin into software catalog
 ---
 
+:::caution Legacy Documentation
+
+This page describes integrating plugins into the Software Catalog using the **old frontend system** patterns (`EntitySwitch`, `EntityLayout`, `EntityLayout.Route`). For the new frontend system, entity page integrations are done using `EntityCardBlueprint` and `EntityContentBlueprint` — see [Common Extension Blueprints](../frontend-system/building-plugins/03-common-extension-blueprints.md).
+
+:::
+
 > This is an advanced use case and currently is an experimental feature. Expect
 > API to change over time
 
diff --git a/docs/plugins/integrating-search-into-plugins.md b/docs/plugins/integrating-search-into-plugins.md
index 7a7201d9e6..c27192e05c 100644
--- a/docs/plugins/integrating-search-into-plugins.md
+++ b/docs/plugins/integrating-search-into-plugins.md
@@ -4,6 +4,12 @@ title: Integrating Search into a plugin
 description: How to integrate Search into a Backstage plugin
 ---
 
+:::caution Legacy Documentation
+
+This section is part of the legacy plugins documentation. The backend search collator patterns described here use the new backend system and are still current. The frontend search experience examples use the old frontend system APIs. For the new frontend system, search result list items are created using `SearchResultListItemBlueprint` — see [Common Extension Blueprints](../frontend-system/building-plugins/03-common-extension-blueprints.md).
+
+:::
+
 The Backstage Search Platform was designed to give plugin developers the APIs
 and interfaces needed to offer search experiences within their plugins, while
 abstracting away (and instead empowering application integrators to choose) the
diff --git a/docs/plugins/internationalization.md b/docs/plugins/internationalization.md
index 2770f76799..7466d17efa 100644
--- a/docs/plugins/internationalization.md
+++ b/docs/plugins/internationalization.md
@@ -4,6 +4,12 @@ title: Internationalization
 description: Documentation on adding internationalization to plugins and apps
 ---
 
+:::caution Legacy Documentation
+
+This section is part of the legacy plugins documentation. The i18n APIs (`createTranslationRef`, `useTranslationRef`) work the same way in both the old and new frontend systems. The "For an application developer" section already shows the new frontend system approach using `createTranslationResource` from `@backstage/frontend-plugin-api`.
+
+:::
+
 ## Overview
 
 The Backstage core function provides internationalization for plugins and apps. The underlying library is [`i18next`](https://www.i18next.com/) with some additional Backstage typescript magic for type safety with keys.
diff --git a/docs/plugins/new-backend-system.md b/docs/plugins/new-backend-system.md
index cf983615b5..e7cef270ab 100644
--- a/docs/plugins/new-backend-system.md
+++ b/docs/plugins/new-backend-system.md
@@ -4,6 +4,12 @@ title: New Backend System
 description: Details of the new backend system
 ---
 
+:::caution Legacy Documentation
+
+This section is part of the legacy plugins documentation. The canonical documentation for the backend system has moved to the [Backend System](../backend-system/index.md) section, which includes more detailed and up-to-date guides for [building plugins and modules](../backend-system/building-plugins-and-modules/01-index.md), [architecture](../backend-system/architecture/01-index.md), and [core services](../backend-system/core-services/01-index.md).
+
+:::
+
 ## Status
 
 The new backend system is released and ready for production use, and many plugins and modules have already been migrated. We recommend all plugins and deployments to migrate to the new system.
diff --git a/docs/plugins/observability.md b/docs/plugins/observability.md
index 7803d90bd5..eab2ed200c 100644
--- a/docs/plugins/observability.md
+++ b/docs/plugins/observability.md
@@ -4,6 +4,12 @@ title: Observability
 description: Adding Observability to Your Plugin
 ---
 
+:::caution Legacy Documentation
+
+This section is part of the legacy plugins documentation. For new backend system logging, see the [Logger](../backend-system/core-services/logger.md) and [Root Logger](../backend-system/core-services/root-logger.md) core service documentation. For health checks, see [Root Health](../backend-system/core-services/root-health.md).
+
+:::
+
 This article briefly describes the observability options that are available to a
 Backstage integrator.
 
diff --git a/docs/plugins/plugin-development.md b/docs/plugins/plugin-development.md
index 5ba1de4a7d..89318e7c5f 100644
--- a/docs/plugins/plugin-development.md
+++ b/docs/plugins/plugin-development.md
@@ -4,6 +4,12 @@ title: Plugin Development
 description: Documentation on Plugin Development
 ---
 
+:::caution Legacy Documentation
+
+This page covers plugin development patterns for the **old frontend system**, including `createPlugin`, `createRoutableExtension`, and `RouteRef` from `@backstage/core-plugin-api`. For the new frontend system equivalents, see [Building Frontend Plugins](../frontend-system/building-plugins/01-index.md) and [Routes](../frontend-system/architecture/36-routes.md).
+
+:::
+
 Backstage plugins provide features to a Backstage App.
 
 Each plugin is treated as a self-contained web app and can include almost any
diff --git a/docs/plugins/plugin-directory-audit.md b/docs/plugins/plugin-directory-audit.md
index 9e82cb6de5..b080463c82 100644
--- a/docs/plugins/plugin-directory-audit.md
+++ b/docs/plugins/plugin-directory-audit.md
@@ -4,6 +4,12 @@ title: Plugin Directory Audit
 description: Details about the process for auditing plugins in the directory
 ---
 
+:::caution Legacy Documentation
+
+This section is part of the legacy plugins documentation. The audit process described here is still current.
+
+:::
+
 ## Audit Process
 
 We have a simple process in place to audit the plugins in the Plugin Directory:
diff --git a/docs/plugins/proxying.md b/docs/plugins/proxying.md
index bd89fc6c36..a764ea64d2 100644
--- a/docs/plugins/proxying.md
+++ b/docs/plugins/proxying.md
@@ -4,6 +4,12 @@ title: Proxying
 description: Documentation on Proxying
 ---
 
+:::caution Legacy Documentation
+
+This section is part of the legacy plugins documentation. The proxy configuration and usage described here applies to both the old and new backend systems. For creating backend plugins and modules, see [Building Backend Plugins and Modules](../backend-system/building-plugins-and-modules/01-index.md).
+
+:::
+
 This page describes how to configure and use the built-in HTTP proxy functionality in your Backstage backend.
 
 ## Overview
diff --git a/docs/plugins/publish-private.md b/docs/plugins/publish-private.md
index d04449d465..b5bb1b937a 100644
--- a/docs/plugins/publish-private.md
+++ b/docs/plugins/publish-private.md
@@ -4,4 +4,10 @@ title: Publish private
 description: Documentation on How to Publish private
 ---
 
+:::caution Legacy Documentation
+
+This section is part of the legacy plugins documentation.
+
+:::
+
 ## TODO
diff --git a/docs/plugins/structure-of-a-plugin.md b/docs/plugins/structure-of-a-plugin.md
index 7cfaca4561..3843359857 100644
--- a/docs/plugins/structure-of-a-plugin.md
+++ b/docs/plugins/structure-of-a-plugin.md
@@ -4,6 +4,12 @@ title: Structure of a Plugin
 description: Details about structure of a plugin
 ---
 
+:::caution Legacy Documentation
+
+This page describes the structure of a plugin for the **old frontend system**. For the new frontend system, see [Building Frontend Plugins](../frontend-system/building-plugins/01-index.md). The general folder structure is similar, but the plugin wiring in `plugin.ts` differs significantly.
+
+:::
+
 Nice, you have a new plugin! We'll soon see how we can develop it into doing
 great things. But first off, let's look at what we get out of the box.
 
diff --git a/docs/plugins/testing.md b/docs/plugins/testing.md
index 1297d516f3..94015700fe 100644
--- a/docs/plugins/testing.md
+++ b/docs/plugins/testing.md
@@ -4,6 +4,12 @@ title: Testing with Jest
 description: Documentation on How to do unit testing with Jest
 ---
 
+:::caution Legacy Documentation
+
+This section is part of the legacy plugins documentation. The general testing principles described here still apply, but for system-specific testing guides, see [Testing Frontend Plugins](../frontend-system/building-plugins/02-testing.md) and [Testing Backend Plugins and Modules](../backend-system/building-plugins-and-modules/02-testing.md).
+
+:::
+
 :::note Note
 
 You may want to consider migrating to Jest 30, to do this, you can follow this guide: [Migrating to Jest 30](../tutorials/jest30-migration.md)
diff --git a/microsite/sidebars.ts b/microsite/sidebars.ts
index ec4585b407..1be013af7d 100644
--- a/microsite/sidebars.ts
+++ b/microsite/sidebars.ts
@@ -412,56 +412,14 @@ export default {
     ),
     sidebarElementWithIndex(
       {
-        label: 'Plugins',
-        description: 'Extend Backstage with custom functionality.',
+        label: 'OpenAPI',
+        description:
+          'Work with OpenAPI specifications and generate clients.',
       },
       [
-        'plugins/index',
-        'plugins/create-a-plugin',
-        'plugins/plugin-development',
-        'plugins/structure-of-a-plugin',
-        'plugins/integrating-plugin-into-software-catalog',
-        'plugins/integrating-search-into-plugins',
-        'plugins/composability',
-        'plugins/internationalization',
-        'plugins/analytics',
-        'plugins/feature-flags',
-        sidebarElementWithIndex(
-          {
-            label: 'OpenAPI',
-            description:
-              'Work with OpenAPI specifications and generate clients.',
-          },
-          [
-            'openapi/01-getting-started',
-            'openapi/generate-client',
-            'openapi/test-case-validation',
-          ],
-        ),
-        sidebarElementWithIndex(
-          {
-            label: 'Backends and APIs',
-            description: 'Build and manage backend services and APIs.',
-          },
-          [
-            'plugins/proxying',
-            'plugins/backend-plugin',
-            'plugins/call-existing-api',
-          ],
-        ),
-        sidebarElementWithIndex(
-          { label: 'Testing', description: 'Testing plugins and modules.' },
-          ['plugins/testing'],
-        ),
-        sidebarElementWithIndex(
-          { label: 'Publishing', description: 'Publishing your plugins.' },
-          [
-            'plugins/publish-private',
-            'plugins/add-to-directory',
-            'plugins/plugin-directory-audit',
-          ],
-        ),
-        'plugins/observability',
+        'openapi/01-getting-started',
+        'openapi/generate-client',
+        'openapi/test-case-validation',
       ],
     ),
     sidebarElementWithIndex(
@@ -719,6 +677,50 @@ export default {
         ),
       ],
     ),
+    sidebarElementWithIndex(
+      {
+        label: 'Plugins (Legacy)',
+        description:
+          'Legacy plugin development documentation for the old frontend system. For new development, see the Frontend System and Backend System sections under Framework.',
+      },
+      [
+        'plugins/index',
+        'plugins/create-a-plugin',
+        'plugins/plugin-development',
+        'plugins/structure-of-a-plugin',
+        'plugins/integrating-plugin-into-software-catalog',
+        'plugins/integrating-search-into-plugins',
+        'plugins/composability',
+        'plugins/internationalization',
+        'plugins/analytics',
+        'plugins/feature-flags',
+        sidebarElementWithIndex(
+          {
+            label: 'Backends and APIs',
+            description: 'Build and manage backend services and APIs.',
+          },
+          [
+            'plugins/proxying',
+            'plugins/backend-plugin',
+            'plugins/call-existing-api',
+          ],
+        ),
+        sidebarElementWithIndex(
+          { label: 'Testing', description: 'Testing plugins and modules.' },
+          ['plugins/testing'],
+        ),
+        sidebarElementWithIndex(
+          { label: 'Publishing', description: 'Publishing your plugins.' },
+          [
+            'plugins/publish-private',
+            'plugins/add-to-directory',
+            'plugins/plugin-directory-audit',
+          ],
+        ),
+        'plugins/observability',
+        'plugins/new-backend-system',
+      ],
+    ),
     sidebarElementWithIndex(
       { label: 'FAQ', description: 'Frequently asked questions and answers.' },
       ['faq/index', 'faq/product', 'faq/technical'],
diff --git a/mkdocs.yml b/mkdocs.yml
index 618f77a59f..2030d87c45 100644
--- a/mkdocs.yml
+++ b/mkdocs.yml
@@ -141,7 +141,11 @@ nav:
           - Locations: 'integrations/google-cloud-storage/locations.md'
       - LDAP:
           - Org Data: 'integrations/ldap/org.md'
-  - Plugins:
+  - OpenAPI:
+      - Schema-first plugins with OpenAPI (Experimental): 'openapi/01-getting-started.md'
+      - Generate a client from your OpenAPI spec: 'openapi/generate-client.md'
+      - Validate your OpenAPI spec against test data: 'openapi/test-case-validation.md'
+  - Plugins (Legacy):
       - Intro to plugins: 'plugins/index.md'
       - Create a Backstage Plugin: 'plugins/create-a-plugin.md'
       - Plugin Development: 'plugins/plugin-development.md'
@@ -151,10 +155,6 @@ nav:
       - Plugin Analytics: 'plugins/analytics.md'
       - Feature Flags: 'plugins/feature-flags.md'
       - Internationalization (i18n): 'plugins/internationalization.md'
-      - OpenAPI:
-          - Schema-first plugins with OpenAPI (Experimental): 'openapi/01-getting-started.md'
-          - Generate a client from your OpenAPI spec: 'openapi/generate-client.md'
-          - Validate your OpenAPI spec against test data: 'openapi/test-case-validation.md'
       - Backends and APIs:
           - Proxying: 'plugins/proxying.md'
           - Backend plugin: 'plugins/backend-plugin.md'
@@ -165,6 +165,7 @@ nav:
           - Publish private: 'plugins/publish-private.md'
           - Add to Directory: 'plugins/add-to-directory.md'
           - Observability: 'plugins/observability.md'
+      - New Backend System: 'plugins/new-backend-system.md'
   - Configuration:
       - Static Configuration in Backstage: 'conf/index.md'
       - Reading Backstage Configuration: 'conf/reading.md'