Issue #3275883: Warn if cron updates are enabled and the site is on an unsupported branch

src/Validator/UnsupportedBranchValidator.php

+<?php
+
+namespace Drupal\automatic_updates\Validator;
+
+use Drupal\automatic_updates\CronUpdater;
+use Drupal\automatic_updates\Event\ReadinessCheckEvent;
+use Drupal\automatic_updates\ProjectInfo;
+use Symfony\Component\EventDispatcher\EventSubscriberInterface;
+
+class UnsupportedBranchValidator implements EventSubscriberInterface {
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function getSubscribedEvents() {
+    return [
+      ReadinessCheckEvent::class => 'checkUpdateVersion',
+    ];
+  }
+
+  public function checkUpdateVersion(ReadinessCheckEvent $event): void {
+    $stage = $event;
+    if (!$stage instanceof CronUpdater || $stage->getMode() !== CronUpdater::DISABLED) {
+      return;
+    }
+    $project_info = new ProjectInfo('drupal');
+    $project_data = $project_info->getProjectInfo();
+
+  }
+
+}