diff -u b/core/modules/page_cache/tests/src/Functional/PageCacheTest.php b/core/modules/page_cache/tests/src/Functional/PageCacheTest.php
--- b/core/modules/page_cache/tests/src/Functional/PageCacheTest.php
+++ b/core/modules/page_cache/tests/src/Functional/PageCacheTest.php
@@ -414,13 +414,13 @@
       $this->assertSession()->statusCodeEquals($code);
       $this->assertSession()->responseHeaderEquals('X-Drupal-Cache', 'MISS');
       $this->drupalGet($content_url);
-      $this->assertResponse($code);
+      $this->assertSession()->statusCodeEquals($code);
       $this->assertEqual($this->drupalGetHeader('X-Drupal-Cache'), 'HIT');
 
       // Setting cache.page.4xx_max_age should invalidate the 4xx cache tag.
       $config->set('cache.page.4xx_max_age', 0)->save();
       $this->drupalGet($content_url);
-      $this->assertResponse($code);
+      $this->assertSession()->statusCodeEquals($code);
       $this->assertEqual($this->drupalGetHeader('X-Drupal-Cache'), 'MISS');
       $this->assertEqual($this->drupalGetHeader('Cache-Control'), 'must-revalidate, no-cache, private');
 
reverted:
--- b/core/modules/system/system.install
+++ a/core/modules/system/system.install
@@ -1457,12 +1457,3 @@
     }
   }
 }
-
-/**
- * Set max-age for 4xx responses to null.
- */
-function system_update_8703() {
-  \Drupal::configFactory()->getEditable('system.performance')
-    ->set('cache.page.4xx_max_age', NULL)
-    ->save(TRUE);
-}
diff -u b/core/modules/system/tests/src/Functional/Update/AddPageCache4xxMaxAgeToSystemPerformanceConfigurationTest.php b/core/modules/system/tests/src/Functional/Update/AddPageCache4xxMaxAgeToSystemPerformanceConfigurationTest.php
--- b/core/modules/system/tests/src/Functional/Update/AddPageCache4xxMaxAgeToSystemPerformanceConfigurationTest.php
+++ b/core/modules/system/tests/src/Functional/Update/AddPageCache4xxMaxAgeToSystemPerformanceConfigurationTest.php
@@ -16,7 +16,7 @@
    */
   protected function setDatabaseDumpFiles() {
     $this->databaseDumpFiles = [
-      __DIR__ . '/../../../../tests/fixtures/update/drupal-8.bare.standard.php.gz',
+      __DIR__ . '/../../../fixtures/update/drupal-8.8.0.bare.standard.php.gz',
     ];
   }
 
@@ -25,14 +25,13 @@
    * configuration.
    */
   public function testUpdate() {
-    \Drupal::configFactory()->getEditable('system.performance')
-      ->set('cache.page.4xx_max_age', NULL)
-      ->save(TRUE);
+    $system_performance = \Drupal::config('system.performance')->get();
+    $this->assertArrayNotHasKey('4xx_max_age', $system_performance['cache']['page'], 'Configuration cache.page.4xx_max_age does not exist in system.performance.');
 
     $this->runUpdates();
 
     $system_performance = \Drupal::config('system.performance')->get();
-    $this->assertTrue(array_key_exists('4xx_max_age', $system_performance['cache']['page']), 'Configuration cache.page.4xx_max_age has been added to system.performance.');
+    $this->assertArrayHasKey('4xx_max_age', $system_performance['cache']['page'], 'Configuration cache.page.4xx_max_age has been added to system.performance.');
   }
 
 }
only in patch2:
unchanged:
--- a/core/modules/system/system.post_update.php
+++ b/core/modules/system/system.post_update.php
@@ -168,3 +168,12 @@ function system_post_update_schema_version_int() {
     }
   }
 }
+
+/**
+ * Set max-age for 4xx responses to null.
+ */
+function system_post_update_set_max_age_4xx() {
+  \Drupal::configFactory()->getEditable('system.performance')
+    ->set('cache.page.4xx_max_age', NULL)
+    ->save(TRUE);
+}