phpBB's user_delete function does not fully clean up notifications for the user being deleted. It deletes only activate user notifications. Line 769 of functions_user.php:

$phpbb_notifications->delete_notifications('notification.type.admin_activate_user', $user_ids);{}

Other notifications often exist, for example notifications to moderate a post or topic. Often a user is deleted because they are posting spam. So I suggest removing all notifications when a user is deleted. Right now this will leave zombie notifications that, in the example, moderators need to manually clean up in the notification manager.