PSA: mastodon.art will soon defederate from tech.lgbt.
Now is a good time to make a backup of the accounts you follow. You can do that in your settings/export and downloading "follows".
You can then use this file to refollow people from a secondary account, without moving your main presence. Or maybe you want to move after defederation already happened. You can import it directly, or use it as reference.
So, what exactly happens when an instance shuts down? Do other servers automatically delete their cache of this instance's data, after a certain period of time? And if so, how long would that typically take?