I run a mastodon server on the fediverse (distributed social networking over activitypub). Mastodon and Pleroma support deletes, but say you post something and someone boosts it and one of their followers sees it on a server your server blocks.

You won't see their replies obviously (split thread) but if you delete the message, it gets deleted from your followers, but won't continue on to the server that your server blocks. So a copy will continue to exist.

I just treat the fediverse as a big chatroom/reddit thread that can never be deleted.