Mastodon has the same fatal flaw. They want to keep your history and relationships hostage so you can’t leave.

You can migrate your relationships to a new Mastodon server.

And while you can't directly transfer the history (the debate over how/whether to do this has gone on for literally years), you can export an archive you can keep locally, and there are tools out there to parse it and convert it to some other form (static website, whatever). Someone's probably written an importer by now, though I'd have to look.