diff --git a/core/core.services.yml b/core/core.services.yml
index b0d4f7cdd16d98460be1e6265293793f97da8b56..12440da791895d0d83dae6f86c799c22da33f575 100644
--- a/core/core.services.yml
+++ b/core/core.services.yml
@@ -1300,10 +1300,6 @@ services:
     tags:
       - { name: event_subscriber }
     arguments: ['@config.factory', '@http_kernel']
-  exception.test_site:
-    class: Drupal\Core\EventSubscriber\ExceptionTestSiteSubscriber
-    tags:
-      - { name: event_subscriber }
   exception.enforced_form_response:
     class: Drupal\Core\EventSubscriber\EnforcedFormResponseSubscriber
     tags:
diff --git a/core/lib/Drupal/Core/EventSubscriber/ExceptionTestSiteSubscriber.php b/core/lib/Drupal/Core/EventSubscriber/ExceptionTestSiteSubscriber.php
deleted file mode 100644
index 2f9745c5c88f2847af9095ff8117de3f340a5eac..0000000000000000000000000000000000000000
--- a/core/lib/Drupal/Core/EventSubscriber/ExceptionTestSiteSubscriber.php
+++ /dev/null
@@ -1,63 +0,0 @@
-<?php
-
-namespace Drupal\Core\EventSubscriber;
-
-use Drupal\Core\Utility\Error;
-use Symfony\Component\HttpKernel\Event\ExceptionEvent;
-
-/**
- * Custom handling of errors when in a system-under-test.
- */
-class ExceptionTestSiteSubscriber extends HttpExceptionSubscriberBase {
-
-  /**
-   * {@inheritdoc}
-   */
-  protected static function getPriority() {
-    return 3;
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  protected function getHandledFormats() {
-    return ['html'];
-  }
-
-  /**
-   * Checks for special handling of errors inside Simpletest.
-   *
-   * @todo The $headers array appears to not actually get used at all in the
-   *   original code. It's quite possible that this entire method is now
-   *   vestigial and can be removed.
-   *
-   * @param \Symfony\Component\HttpKernel\Event\ExceptionEvent $event
-   *   The event.
-   */
-  public function on500(ExceptionEvent $event) {
-    $exception = $event->getThrowable();
-    $error = Error::decodeException($exception);
-
-    $headers = [];
-
-    // When running inside the testing framework, we relay the errors
-    // to the tested site by the way of HTTP headers.
-    if (DRUPAL_TEST_IN_CHILD_SITE && !headers_sent() && (!defined('SIMPLETEST_COLLECT_ERRORS') || SIMPLETEST_COLLECT_ERRORS)) {
-      // $number does not use drupal_static as it should not be reset
-      // as it uniquely identifies each PHP error.
-      static $number = 0;
-      $assertion = [
-        $error['@message'],
-        $error['%type'],
-        [
-          'function' => $error['%function'],
-          'file' => $error['%file'],
-          'line' => $error['%line'],
-        ],
-      ];
-      $headers['X-Drupal-Assertion-' . $number] = rawurlencode(serialize($assertion));
-      $number++;
-    }
-  }
-
-}