If you or someone else has an archive of your data, then you can seamlessly port your account somewhere else. With Mastodon, I'm not sure there's an established flow for downloading your full account data, and you have to have your old server cooperate and redirect your user page to your page on your new server.
Look at Shitter and Reddit: they just turned off API access and introduced heavy rate limits to webpage loads. Good luck scraping your account details with that.
Enshittification is a thing with ALL commercial services. And eventually BlueSky will have their "The sky is falling! Crank the money extraction lever." And I'd move that timeframe up a LOT if they took VC money.
Feces, err, uhmm VCs want their hockeystick growth to be a hockeystick. They want their 30x , 50x, or 100x.
The handful of moderately large instances that have shut down rhat I'm aware of gave long notices, in a couple of cases over a year, before actually going offline.
The only notable counterexample I remember was BitcoinHackers.org shutting down suddenly with a note saying "haha look at how easy it is for mastodon instances to shut down go use nostr", making it a self-fulfilling prophecy in that case. If you have other examples I would like to know about them.
I've personally had to migrate instances twice. The first time because the instance suddenly became nonfunctional, and the administrator went AWOL. The second time, I discovered that the incompetent admin had silently enabled auto-deletion of data including posts and direct messages.
Now I'm finally on mastodon.social, which wasn't open for new users the first two times that I needed an instance.
You can if the server is operational. If the server not operational and cooperative, you can't. And you can't migrate your posts, only your followers.
> Server shutdowns are rare
Not rare enough though.