core/core.services.yml                             |  4 +--
 core/lib/Drupal/Core/Routing/BcRoute.php           | 36 +++++++++++++++++++++-
 .../RestResourceGetRouteProcessorBC.php            | 25 ++-------------
 .../EntityResource/EntityResourceTestBase.php      |  9 ++----
 4 files changed, 42 insertions(+), 32 deletions(-)

diff --git a/core/core.services.yml b/core/core.services.yml
index 9b64819..d848df3 100644
--- a/core/core.services.yml
+++ b/core/core.services.yml
@@ -938,8 +938,8 @@ services:
       # The HTTP method route filter must run very early: it removes any routes
       # whose requirements do not allow the HTTP method of the current request.
       # Throws a 405 if no routes match the current request's HTTP method.
-      # (If it runs before content_type_header_matcher, it can ensure that that
-      # only receives routes which can have a Content-Type request header.)
+      # (If it runs before content_type_header_matcher, it can ensure that only
+      # receives routes which can have a Content-Type request header.)
       - { name: route_filter, priority: 10 }
   content_type_header_matcher:
     class: Drupal\Core\Routing\ContentTypeHeaderMatcher
diff --git a/core/lib/Drupal/Core/Routing/BcRoute.php b/core/lib/Drupal/Core/Routing/BcRoute.php
index e886931..a0af044 100644
--- a/core/lib/Drupal/Core/Routing/BcRoute.php
+++ b/core/lib/Drupal/Core/Routing/BcRoute.php
@@ -14,7 +14,7 @@
  * - have an accompanying outbound route processor, that overwrites this empty
  *   route definition with the redirected route's definition.
  *
- * F.e. see \Drupal\rest\RouteProcessor\RestResourceGetRouteProcessorBC.
+ * @see \Drupal\rest\RouteProcessor\RestResourceGetRouteProcessorBC
  */
 class BcRoute extends Route {
 
@@ -26,4 +26,38 @@ public function __construct() {
     $this->setOption('bc_route', TRUE);
   }
 
+  /**
+   * {@inheritdoc}
+   */
+  public function serialize() {
+    // Ensure this route definition is as empty as possible when it gets stored.
+    return serialize([
+      'path' => '',
+      'host' => '',
+      'defaults' => [],
+      'requirements' => [],
+      'options' => [],
+      'schemes' => [],
+      'methods' => [],
+    ]);
+  }
+
+  /**
+   * Inherit definition of target route, for use in outbound route processors.
+   *
+   * @param \Symfony\Component\Routing\Route $target_route
+   *   The route that is the redirect target.
+   *
+   * @return $this
+   */
+  public function inheritDefinitionFromTargetRoute(Route $target_route) {
+    $this->setPath($target_route->getPath());
+    $this->setDefaults($target_route->getDefaults());
+    $this->setRequirements($target_route->getRequirements());
+    $this->setOptions($target_route->getOptions());
+    $this->setHost($target_route->getHost());
+    $this->setSchemes($target_route->getSchemes());
+    $this->setMethods($target_route->getMethods());
+  }
+
 }
diff --git a/core/modules/rest/src/RouteProcessor/RestResourceGetRouteProcessorBC.php b/core/modules/rest/src/RouteProcessor/RestResourceGetRouteProcessorBC.php
index cf55326..41052e2 100644
--- a/core/modules/rest/src/RouteProcessor/RestResourceGetRouteProcessorBC.php
+++ b/core/modules/rest/src/RouteProcessor/RestResourceGetRouteProcessorBC.php
@@ -4,6 +4,7 @@
 
 use Drupal\Core\Render\BubbleableMetadata;
 use Drupal\Core\RouteProcessor\OutboundRouteProcessorInterface;
+use Drupal\Core\Routing\BcRoute;
 use Drupal\Core\Routing\RouteProviderInterface;
 use Symfony\Component\Routing\Route;
 
@@ -49,32 +50,12 @@ public function processOutbound($route_name, Route $route, array &$parameters, B
     // route definitions that are as empty as possible, plus an outbound route
     // processor.
     // @see \Drupal\rest\Plugin\ResourceBase::routes()
-    if ($route_name_parts[0] === 'rest' && $route_name_parts[count($route_name_parts) - 2] === 'GET' && in_array($route_name_parts[count($route_name_parts) - 1], $this->serializerFormats, TRUE)) {
+    if ($route instanceof BcRoute && $route_name_parts[0] === 'rest' && $route_name_parts[count($route_name_parts) - 2] === 'GET' && in_array($route_name_parts[count($route_name_parts) - 1], $this->serializerFormats, TRUE)) {
       array_pop($route_name_parts);
       $redirected_route_name = implode('.', $route_name_parts);
       @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));
+      $route->inheritDefinitionFromTargetRoute($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/rest/tests/src/Functional/EntityResource/EntityResourceTestBase.php b/core/modules/rest/tests/src/Functional/EntityResource/EntityResourceTestBase.php
index 48bfc12..a9164b6 100644
--- a/core/modules/rest/tests/src/Functional/EntityResource/EntityResourceTestBase.php
+++ b/core/modules/rest/tests/src/Functional/EntityResource/EntityResourceTestBase.php
@@ -659,13 +659,8 @@ public function testGet() {
     $other_format = static::$format === 'json' ? 'xml' : 'json';
     $bc_route_other_format = Url::fromRoute('rest.entity.' . static::$entityTypeId . '.GET.' . $other_format, $url->getRouteParameters(), $url->getOptions());
     $bc_route_other_format->setUrlGenerator($this->container->get('url_generator'));
-    try {
-      $bc_route_other_format->toString(TRUE);
-      $this->assertTrue(FALSE);
-    }
-    catch (RouteNotFoundException $e) {
-      $this->assertTrue(TRUE);
-    }
+    $this->setExpectedException(RouteNotFoundException::class);
+    $bc_route_other_format->toString(TRUE);
   }
 
   /**