last week i was in a conversation with a few people about social media. i guess they were finally leaving xitter and wanted to know where to go. cohost came up and they all made accounts immediately. then i mentioned mastodon and was immediately rebuffed because "sometimes those instances shut down"
I don't think that's even desirable and also legally questionable. But anyways, these posts are not gone with an instance shutting down and thus I don't really see a problem. You can always add a link to a mirrow of those old posts in your profile.
Can you do that with a letter once it is send? And the instance admin of the mirroring server can delete posts if that is legally required for some reason.
And how would that even work technically? Bulk import posts and spam other instances with mass updates? That would immediately detected as a spam-wave and blocked. And back dating technically new messages is also not exactly a great thing to allow.
Other implementations of nomadic identity like Hubzilla get around this by letting you run two accounts in parallel and syncing them from your main account, but they will also not back-port old messages before you linked up the secondary account.
Basically anyone with some experience with federated systems agrees that importing old messages in bulk on account migration will never happen, and I don't really see an issue with that, since messages are not lost.
This should hopefully get better over time as some instances stick around for longer. You'll be able to point to instances that have stuck around for a while, which means they'll probably stick around for some time longer. The problem right now is that the fediverse and many instances are still young, and something that started yesterday is not too likely to still exist tomorrow.
i just have such a hard time wrapling my head around why the fedi is under that level of scrutiny to begin with while everyone assumed cohost would be forever. i had an account there but stopped using it years ago because half the time i tried to log in it was down! come october there will be a plethora of mastodon instances that both predate and have outlived cohost