.../settings_tray/settings_tray.routing.yml        |  7 +++
 .../settings_tray/settings_tray.services.yml       |  9 +++
 .../BlockEntityOffCanvasFormRouteProcessorBC.php   | 65 ++++++++++++++++++++++
 .../tests/src/Functional/BcRoutesTest.php          | 33 +++++++++++
 4 files changed, 114 insertions(+)

diff --git a/core/modules/settings_tray/settings_tray.routing.yml b/core/modules/settings_tray/settings_tray.routing.yml
index d1040b3..fd340e5 100644
--- a/core/modules/settings_tray/settings_tray.routing.yml
+++ b/core/modules/settings_tray/settings_tray.routing.yml
@@ -6,3 +6,10 @@ entity.block.settings_tray_form:
   requirements:
     _permission: 'administer blocks'
     _access_block_plugin_has_settings_tray_form: 'TRUE'
+
+# Deprecated.
+# @see entity.block.settings_tray_form
+# @see \Drupal\settings_tray\RouteProcessor\BlockEntityOffCanvasFormRouteProcessorBC
+# @todo Remove in Drupal 9.0.0.
+entity.block.off_canvas_form:
+  path: ''
diff --git a/core/modules/settings_tray/settings_tray.services.yml b/core/modules/settings_tray/settings_tray.services.yml
index a11a1d4..0457101 100644
--- a/core/modules/settings_tray/settings_tray.services.yml
+++ b/core/modules/settings_tray/settings_tray.services.yml
@@ -3,3 +3,12 @@ services:
     class: Drupal\settings_tray\Access\BlockPluginHasSettingsTrayFormAccessCheck
     tags:
       - { name: access_check, applies_to: _access_block_plugin_has_settings_tray_form }
+
+  # BC layers.
+  # @todo Remove in Drupal 9.0.0.
+  settings_tray.route_processor_off_canvas_form_bc:
+    class: \Drupal\settings_tray\RouteProcessor\BlockEntityOffCanvasFormRouteProcessorBC
+    arguments: ['@router.route_provider']
+    public: false
+    tags:
+      - { name: route_processor_outbound }
diff --git a/core/modules/settings_tray/src/RouteProcessor/BlockEntityOffCanvasFormRouteProcessorBC.php b/core/modules/settings_tray/src/RouteProcessor/BlockEntityOffCanvasFormRouteProcessorBC.php
new file mode 100644
index 0000000..48935ae
--- /dev/null
+++ b/core/modules/settings_tray/src/RouteProcessor/BlockEntityOffCanvasFormRouteProcessorBC.php
@@ -0,0 +1,65 @@
+<?php
+
+namespace Drupal\settings_tray\RouteProcessor;
+
+use Drupal\Core\Render\BubbleableMetadata;
+use Drupal\Core\RouteProcessor\OutboundRouteProcessorInterface;
+use Drupal\Core\Routing\RouteProviderInterface;
+use Symfony\Component\Routing\Route;
+
+/**
+ * Processes the Block entity off-canvas form BC route.
+ *
+ * @internal
+ */
+class BlockEntityOffCanvasFormRouteProcessorBC implements OutboundRouteProcessorInterface {
+
+  /**
+   * The route provider.
+   *
+   * @var \Drupal\Core\Routing\RouteProviderInterface
+   */
+  protected $routeProvider;
+
+  /**
+   * Constructs a BlockEntityOffCanvasFormRouteProcessorBC object.
+   *
+   * @param \Drupal\Core\Routing\RouteProviderInterface $route_provider
+   *   The route provider.
+   */
+  public function __construct(RouteProviderInterface $route_provider) {
+    $this->routeProvider = $route_provider;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function processOutbound($route_name, Route $route, array &$parameters, BubbleableMetadata $bubbleable_metadata = NULL) {
+    if ($route_name === 'entity.block.off_canvas_form') {
+      $redirected_route_name = 'entity.block.settings_tray_form';
+      @trigger_error(sprintf("The '%s' route is deprecated since version 8.5.x and will be removed in 9.0.0. Use the '%s' route instead.", $route_name, $redirected_route_name), E_USER_DEPRECATED);
+      static::overwriteRoute($route, $this->routeProvider->getRouteByName($redirected_route_name));
+    }
+  }
+
+  /**
+   * Overwrites one route's metadata with the other's.
+   *
+   * @param \Symfony\Component\Routing\Route $target_route
+   *   The route whose metadata to overwrite.
+   * @param \Symfony\Component\Routing\Route $source_route
+   *   The route whose metadata to read from.
+   *
+   * @see \Symfony\Component\Routing\Route
+   */
+  protected static function overwriteRoute(Route $target_route, Route $source_route) {
+    $target_route->setPath($source_route->getPath());
+    $target_route->setDefaults($source_route->getDefaults());
+    $target_route->setRequirements($source_route->getRequirements());
+    $target_route->setOptions($source_route->getOptions());
+    $target_route->setHost($source_route->getHost());
+    $target_route->setSchemes($source_route->getSchemes());
+    $target_route->setMethods($source_route->getMethods());
+  }
+
+}
diff --git a/core/modules/settings_tray/tests/src/Functional/BcRoutesTest.php b/core/modules/settings_tray/tests/src/Functional/BcRoutesTest.php
new file mode 100644
index 0000000..6750b1c
--- /dev/null
+++ b/core/modules/settings_tray/tests/src/Functional/BcRoutesTest.php
@@ -0,0 +1,33 @@
+<?php
+
+namespace Drupal\Tests\settings_tray\Functional;
+
+use Drupal\Core\Url;
+use Drupal\Tests\BrowserTestBase;
+
+/**
+ * Tests Settings Tray BC routes.
+ *
+ * @group settings_tray
+ * @group legacy
+ */
+class BcRoutesTest extends BrowserTestBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  public static $modules = [
+    'settings_tray',
+  ];
+
+  /**
+   * @expectedDeprecation The 'entity.block.off_canvas_form' route is deprecated since version 8.5.x and will be removed in 9.0.0. Use the 'entity.block.settings_tray_form' route instead.
+   */
+  public function testOffCanvasFormRouteBc() {
+    $block = $this->placeBlock('system_powered_by_block');
+    $url_for_current_route = Url::fromRoute('entity.block.settings_tray_form', ['block' => $block->id()])->toString(TRUE)->getGeneratedUrl();
+    $url_for_bc_route = Url::fromRoute('entity.block.off_canvas_form', ['block' => $block->id()])->toString(TRUE)->getGeneratedUrl();
+    $this->assertSame($url_for_current_route, $url_for_bc_route);
+  }
+
+}