My observations having gone through some dead instances in these past few years:

The community stays derelict after an instance dies, so I’d suggest adding something to its name so others may know. Maybe something like [moved] or [instance dead/dying]. Also restricting posting before the instance dies might help avoid potential future spam.

And afaik, the images die with the instance unless the instance(s) that federated with it kept a local mirror. But the text contents of a post remain accessible if they got federated.

Posts and communities from a dead instance cannot be further federated, bar from a few cases. First, afaik specific for softwares with functions to follow users, if an user on an instance A follows someone on instance B, and this user from the instance B interacts with a post of the dead instance that was fetched before dying, the interaction pulls the text skeleton of the post and the skeleton of the community to the instance A. Second possibility, which works for most instances afaik, if someone from an still active instance had commented on a dead instance’s post, you can get the comment’s fediverse link (e.g. rainbow star on Lemmy, “original URL” on Mbin) and search for it on your local instance search bar/function, which pulls the skeleton of the post and community. Third possibility, similar to the first but also needing boost support, boosting a post from a dead instance or a comment in such a post.