alt Hacker Newshistoric posts from the known network and (sometimes media, instance setting) are cached on your own instance in ActivityPub. interactions travel across the known network graph. if an instance vanished forever, overnight, there is at least an imprint of it across the network, albeit instance specific. that may be by design, there are jurisdictions that have people complying with laws and things. not sure how the ecosystems you mention deal with that in particular
That doesn’t answer the point I’m making. If the instance your account was made on explodes, YOU lose your social graph, wether some of your posts survive cached elsewhere is not relevant, your account is gone, and so are your connections.
You have no way to prove an account made after the original instance went down belongs to someone, that’s the issue with federated systems.
As for content moderation, in nostr relay operators such as nostr.build handle legal takedowns on a daily basis, SSB is a little trickier since it’s mostly p2p but pubs are still able to control what flows through them to some degree.