.../src/Plugin/DisplayVariant/BlockPageVariant.php |  7 +++++++
 .../system/src/Plugin/Block/SystemMainBlock.php    | 23 ----------------------
 2 files changed, 7 insertions(+), 23 deletions(-)

diff --git a/core/modules/block/src/Plugin/DisplayVariant/BlockPageVariant.php b/core/modules/block/src/Plugin/DisplayVariant/BlockPageVariant.php
index 798805e..7b67093 100644
--- a/core/modules/block/src/Plugin/DisplayVariant/BlockPageVariant.php
+++ b/core/modules/block/src/Plugin/DisplayVariant/BlockPageVariant.php
@@ -142,6 +142,13 @@ public function build() {
           $messages_block_displayed = TRUE;
         }
         $build[$region][$key] = $this->blockViewBuilder->view($block);
+
+        // The main content block cannot be cached: it is a placeholder for the
+        // render array returned by the controller. It should be rendered as-is,
+        // with other placed blocks "decorating" it.
+        if ($block_plugin instanceof MainContentBlockPluginInterface) {
+          unset($build[$region][$key]['#cache']['keys']);
+        }
       }
       if (!empty($build[$region])) {
         // \Drupal\block\BlockRepositoryInterface::getVisibleBlocksPerRegion()
diff --git a/core/modules/system/src/Plugin/Block/SystemMainBlock.php b/core/modules/system/src/Plugin/Block/SystemMainBlock.php
index 3d2d45e..2bb2479 100644
--- a/core/modules/system/src/Plugin/Block/SystemMainBlock.php
+++ b/core/modules/system/src/Plugin/Block/SystemMainBlock.php
@@ -9,7 +9,6 @@
 
 use Drupal\Core\Block\BlockBase;
 use Drupal\Core\Block\MainContentBlockPluginInterface;
-use Drupal\Core\Form\FormStateInterface;
 
 /**
  * Provides a 'Main page content' block.
@@ -42,26 +41,4 @@ public function build() {
     return $this->mainContent;
   }
 
-  /**
-   * {@inheritdoc}
-   */
-  public function buildConfigurationForm(array $form, FormStateInterface $form_state) {
-    $form = parent::buildConfigurationForm($form, $form_state);
-
-    // The main content block is never cacheable, because it may be dynamic.
-    $form['cache']['#disabled'] = TRUE;
-    $form['cache']['#description'] = $this->t('This block is never cacheable, it is not configurable.');
-    $form['cache']['max_age']['#value'] = 0;
-
-    return $form;
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  public function getCacheMaxAge() {
-    // The main content block is never cacheable, because it may be dynamic.
-    return 0;
-  }
-
 }