I think the only right answer here is: it depends on use case.

Not everyone wants everything archived for potential future use. Some people not only want, but are also mandated by law to have an appropriate archive of their communications.

My point is that we should be able to figure out options for both and anything in between, where user has control over it. This is the important part. As a user, I used to be able to have some modicum of control over what happens on the machines I own and the data I send to others. Over the years, that control was slowly wrangled from the population under various guises ( cloud, convenience and so on ).

But instead of discussing protocols that would, hopefully, behave in deterministic and predictable ways, we discuss platforms and their pinkyswear promises not to do something.

It is a lot to ask these days, because a lot of people got really used to 'easy' and 'free' internet. And that is before we get to the technical issues you mention.

99.9%+ plus of the time, no one really cares about most things that most people post on the web. And you can probably get it deleted from something like The Wayback Machine. But if you actually post something that attracts attention, it's probably gotten copied and is out of the bottle without even any real mechanism to request its deletion.

Not anything with a reasonable expectation of privacy, which would either only be federated to the nodes of the people who are supposed to see it or not at all. Decentralized platforms are identical to the vast majority of the internet in their archivability, the distinction is in their administration.