The point here is that those stay server-side and are thrown away together with the messages when they get deleted. Whatever's sent to the client can be stored

The point here is that those stay server-side and are thrown away together with the messages when they get deleted. Whatever's sent to the client can be stored by him/her indefinitely. BTW. There is probably a time limit: if this is really a MAC, then the key used to generate it is most likely rotated once in a while.