Just because it’s not possible to be sure something got deleted, doesn’t mean that we shouldn’t be designing systems that are immutable. Users should be able to request their own content be deleted, either with a new message, or setting the expiration. Well behaved relays should respect that, and actually delete instead of just leaving it up to the client which might only get a partial feed and miss the delete message.

There will DEFINATELY be relays and bots which craw the network and archive things. But in the normal flow of the app, most users, will stop seeing and hosting content the original poster wanted to remove. It will take it out of normal circulation on the nostrverse and while not perfect, is important.