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.