Asklemmy

Each Lemmy instance keeps a copy of posts and contents made in communities followed by its users, so even though an instance is decommissioned, the content lives on on other instance.

For example my instance was off for a month (I was moving out, setting up my NAS was my last priority) and during this time my posts and comments were still available on other instances.

As for users unable to login on decommissioned instances, unfortunately all they can do is create an account on another instance.

The local copies cease to exist online. Remote copies on other instances stay. There's a number of communities I subscribed to that no longer exist independently, but which I can still see on my old instance, for example.

The content shouldn't be gone as long as that server was federated with somewhere?

It will be more difficult to get to IMO - you'll have to either use a search engine, or query the lemmy resolve object (rs) api method on random instances until you hit one that has the federated content AFAICT.

Images are definitely lost though unless they're uploaded to a third party that is still operational

The Elder Lord Farzghiqud absorbs the data into his eternal glorious form where it will be analyzed and used for the eventual enslavement of your species.

All hail Farzghiqud!