Send a notification to the admin and the user when a registration has been deleted.

You could approach this similarly to how you did it when a registrant was created in 💬 How to send a notification e-mail to the admin when a registration has been made? Fixed , but instead use the YOUR_MODULE_registrant_delete hook.

Thanks for the helpful hint.
I have now implemented the whole thing like this:

First I added a notification_type:

function terminbuchung_recurring_events_registration_notification_types_alter(array &$notification_types) {
    'delete_registration_notification' => [
      'name' => t('Delete Registration Notification'),
      'description' => t('Send an email to an admin when someone deletes a registration for an event'),
    ],        
  ];
}

Then the function:

function terminbuchung_registrant_delete(EntityInterface $entity) {
  $key = 'delete_registration_notification';
  $to = 'YOURMAIL';
  $params = [
    'registrant' => $entity,
  ];
  $currentDateTime = new DateTime();
  $dateString = $params['registrant']->field_reg_datum_start->value;
  $dateTime = new DateTime($dateString);
    
  if ($currentDateTime <= $dateTime) {
    $mail = \Drupal::service('plugin.manager.mail');
    $mail->mail('recurring_events_registration', $key, $to, \Drupal::languageManager()->getDefaultLanguage()->getId(), $params);
  }
}

By querying the date, I prevent an e-mail from being sent when past registrations are deleted. I think this is necessary so that an e-mail is not sent to all deleted registrations when an event series is deleted.

Great stuff! I am glad you got it to work.

I'll close this out as it seems to be working for you.

Automatically closed - issue fixed for 2 weeks with no activity.