designtopack/public/site/plugins/notifications/routes/mark-as-read.php

<?php

return [
  'pattern' => '(:all)mark-as-read.json',
  'method' => 'POST',
  'action'  => function () {
      $json = file_get_contents('php://input');
      $data = json_decode($json);
      $user = kirby()->user($data->userUuid);

      $newNotifications = $user->readNotification($data->group, $data->notificationId);

      return $newNotifications;
  }
];