For communities that have been federated, their historical data, posts, comments, etc should still be available on other instances. So in a sense they're already archived.

Besides that, we have a backup / restore guide on the join-lemmy docs that show how to backup your DB, which every server should be doing.

Every instance automatically archives content from other instances that it federates with (though this doesnt include images). An archive sounds like a good idea, but we certainly dont have the capacity to take on another project. It would have to be done by someone from the community.