While I understand the concept of federation I’m unclear as to what it actually entails at a technical level. When lemm.ee eventually shuts down, will all of that user content (posts, comments, votes, etc) cease to exist? Is it mirrored on other instances?
That’s pretty much it.
Every instance that was subscribed to those communities will retain their local copy of the posts/comments and remain visible. You can even still comment on them, but only users on your instance will see them (i.e. without the home instance for the community being online, they won’t federate beyond your local instance).
The only hiccup is image posts. Different instances have different configs for creating local thumbnails of images, and community icons/banners aren’t cached, so it’ll mostly be text-only copies of the content.
And any comments or posts to other communities by
.eeusers that have images hosted on.eewill be broken.