I've noticed that new posts aren't coming through from the subreddits, and newly requested subreddits aren't getting the automated response either. Seems to have stopped about 3 days ago.
Apologies if this is known/planned, but seeing as I couldn't see any announcement about it I figured I ought to report it.
Hmm, seems like the Lemmit scripts are already using old.reddit.com. As linked by BlackEco above: https://gitlab.com/sab_from_earth/lemmit/-/blob/develop/src/reddit/reader.py#L46. The format is a little different, though; the Lemmit scripts use .../r/subredditname/.rss whereas that post you linked to says .../r/subredditname.rss.
Also, just FYI, your mention of the admin doesn't seem to have worked. If it doesn't come up in the suggestion list to select, I think it works if you just create a link: [@admin@lemmit.online](https://lemmit.online/u/admin). That's all that selecting a user from the suggestion popup seems to do, anyway.