If you search back far enough on some lemmy instances that have defederated others, you'll find ghosts of old content from those defederated servers, but it's all local to whatever instance you're viewing it on. A large amount of the content from the server that went down should also exist on the servers that server was federated with.

These lemmy instances have got to start running out of storage though, I haven't heard of any kind of automated purging. I'd bet someone somewhere is already working on an archive lemmy.

Same as when one of the big name hosting companies takes a site down. You hope it's archived, and if it was important enough to you, hopefully you saved it to your personal server.

What you're describing is a major benefit of federation. Any site can be taken down. But when a federated server goes down it's because the site owner exercised their control over their own data. If Google or Amazon takes a site down, you lose your data, but they keep copies to use however they want.

So this pretty much happened with feddit.de

The admin took off for a business trip and his backup forgot the password and locked himself out. Now the admin seems to have vanished and the server shit itself, making all previously posted pictures unavailable.

The solution is that a bunch of lemmings are forming a "Verein" (Kind of Like a club I suppose?) and will build an entirely new German instance.

So, it's not pretty.