Issue #3274938 by xjm, nod_: Remove deprecated public Backbone and Underscore libraries

core/core.libraries.yml

 # All libraries are defined in alphabetical order.
 
-backbone:
-  remote: https://github.com/jashkenas/backbone
-  version: "1.4.0"
-  license:
-    name: MIT
-    url: https://raw.githubusercontent.com/jashkenas/backbone/1.4.0/LICENSE
-    gpl-compatible: true
-  js:
-    assets/vendor/backbone/backbone-min.js: { weight: -19, minified: true }
-  dependencies:
-    - core/underscore
-  deprecated: The %library_id% asset library is deprecated in Drupal 9.4.0 and will be removed in Drupal 10.0.0.
-
 internal.backbone:
   # Internal library. Do not depend on it outside core nor add new core usage.
   # The library will be removed as soon as the following issues are fixed:
@@ -1091,17 +1078,6 @@ tabbable.jquery.shim:
     - core/tabbable
     - core/jquery
 
-underscore:
-  remote: https://github.com/jashkenas/underscore
-  version: "1.13.2"
-  license:
-    name: MIT
-    url: https://raw.githubusercontent.com/jashkenas/underscore/1.13.2/LICENSE
-    gpl-compatible: true
-  js:
-    assets/vendor/underscore/underscore-min.js: { weight: -20, minified: true }
-  deprecated: The %library_id% asset library is deprecated in Drupal 9.4.0 and will be removed in Drupal 10.0.0.
-
 internal.underscore:
   # Internal library. Do not depend on it outside core nor add new core usage.
   # The library will be removed as soon as the following issues are fixed:

core/scripts/js/vendor-update.js

@@ -79,10 +79,6 @@ const assetsFolder = `${coreFolder}/assets/vendor`;
       library: 'internal.backbone',
       files: ['backbone.js', 'backbone-min.js', 'backbone-min.map'],
     },
-    // Only used to update the version number of the deprecated library.
-    {
-      pack: 'backbone',
-    },
     {
       pack: 'css.escape',
       folder: 'css-escape',
@@ -179,10 +175,6 @@ const assetsFolder = `${coreFolder}/assets/vendor`;
       library: 'internal.underscore',
       files: ['underscore-min.js', 'underscore-min.js.map'],
     },
-    // Only used to update the version number of the deprecated library.
-    {
-      pack: 'underscore',
-    },
     {
       pack: 'loadjs',
       files: [{ from: 'dist/loadjs.min.js', to: 'loadjs.min.js' }],

core/tests/Drupal/KernelTests/Core/Asset/DeprecatedBackboneTest.php

-<?php
-
-namespace Drupal\KernelTests\Core\Asset;
-
-use Drupal\KernelTests\KernelTestBase;
-
-/**
- * Checks the deprecation status of Backbone.
- *
- * @group Asset
- * @group legacy
- */
-class DeprecatedBackboneTest extends KernelTestBase {
-
-  /**
-   * The library discovery service.
-   *
-   * @var \Drupal\Core\Asset\LibraryDiscoveryInterface
-   */
-  protected $libraryDiscovery;
-
-  /**
-   * {@inheritdoc}
-   */
-  protected function setUp():void {
-    parent::setUp();
-    $this->libraryDiscovery = $this->container->get('library.discovery');
-  }
-
-  /**
-   * Tests that the Backbone library is deprecated.
-   */
-  public function testBackboneDeprecation() {
-    $this->libraryDiscovery->getLibraryByName('core', 'backbone');
-    $this->expectDeprecation("The core/backbone asset library is deprecated in Drupal 9.4.0 and will be removed in Drupal 10.0.0.");
-  }
-
-}