diff -u b/core/modules/update/lib/Drupal/update/UpdateSettingsForm.php b/core/modules/update/lib/Drupal/update/UpdateSettingsForm.php
--- b/core/modules/update/lib/Drupal/update/UpdateSettingsForm.php
+++ b/core/modules/update/lib/Drupal/update/UpdateSettingsForm.php
@@ -48,6 +48,7 @@
       '#type' => 'checkbox',
       '#title' => t('Enable Update module system notifications'),
       '#default_value' => $config->get('check.update_system_notifications'),
+      '#description' => t('Messages about available updates will not be displayed if this checkbox is unchecked. Checked by default.'),
     );
 
     $notification_emails = $config->get('notification.emails');
diff -u b/core/modules/update/update.module b/core/modules/update/update.module
--- b/core/modules/update/update.module
+++ b/core/modules/update/update.module
@@ -99,7 +99,7 @@
  * Implements hook_page_build().
  */
 function update_page_build() {
-  if (arg(0) == 'admin' && user_access('administer site configuration') && config('update.settings')->get('check.update_system_notifications')) {
+  if (arg(0) == 'admin' && user_access('administer site configuration') && Drupal::config('update.settings')->get('check.update_system_notifications')) {
     switch (current_path()) {
       // These pages don't need additional nagging.
       case 'admin/appearance/update':