designtopack/public/site/plugins/notifications/user-methods/delete.php

<?php

return function($project, $notificationId) { 
  foreach (kirby()->users() as $user) {
    try {
      $notifications = $user->notifications()->isNotEmpty() 
          ? Yaml::decode($user->notifications()->value())
          : [];          
          
      foreach ($notifications as $key => $notification) {
        if ($notification['id'] === $notificationId) {
          unset($notifications[$key]);
        }
      }
  
      $user->update([
        'notifications' => Yaml::encode(array_values($notifications))
      ]);
    } catch (\Throwable $th) {
      throw new Exception("Error updating notifications: " . $th->getMessage() . ' line ' . $th->getLine(), 1);
    }
  }
};