Currently it is not possible to add a new notification method (in my case telegram) in an extension, which informs the user on every new post, while the other methods (email and board) notify only again, when the forum was visited in the meantime.

Although get_notified_users() is called for every notification method, the result is not treated method specific. I can overwrite get_notified_users() in the board and email method, so that they return nothing. But then the users will not only receive telegram notifications on each new post but also email notifications, which they probably do not want.

My proposal would be to treat the result of get_notified_users() in such a way, that it only effects the notification method, where it was called for.

That means, inside the loop, where the notification manager calls 
$method->get_notified_users(), instead of unsetting the user completely, only the corresponding notification method of the user should be unset.