Hello.
I found that if inside General tab you choose "Request Manual Accounts Deletion", It will not send notification email to Administrator.
I was forced to update your onRequestdelete() function by adding ActivitiesHelper::sendManualRequestMail();
After that emails are sending to Administrator.
check code below:
function onRequestdelete() {
// Send admin notification email when a user send manual deletion request
$app = JFactory::getApplication();
$user = JFactory::getUser();
// send to activities
ActivitiesHelper::insertActivity($user, 'request delete', 'pending', 'joomla');
ActivitiesHelper::sendManualRequestMail();
$app->enqueueMessage(JText::_('COM_JAGDPR_REQUEST_DELETE_MESSAGE_SENT'));
$app->redirect(JRoute::_('index.php?option=com_jagdpr'));
jexit();
}