There is a open issue , the developers talked about it but no one pulled the trigger on it, this can be implemented on the client in a way that is pretty good (you could create client side backups using something like dropbox). We could open more issues on the various lemmy clients and maybe even piefed which seems to prioritize feature development in a way that might be better then lemmy developers currently. you can already read the comments on piefed and then subscribe to the posts and incrementally read new comments.