I have searched for various backup newsgroup access for those times when Eternal September goes down or has technical issues.
For a long time Google groups was the defacto fall back, but they did nothing to filter the onslaught of spam, and finally just gave up.
Polytech forum uses content from this group to juice up their web forums, but they are slow to update, and practice a high degree of censorship.
Narkive backs up many groups, but they can be difficult navigate, you sometimes get old results, and it is slow to update. As near as I can tell you can't post.
A while back I discovered a web portal that mirrors all the Usenet groups I read, doesn't appear to censor any legitimate (or semi legitimate) post, and filters out all the SPAM. Mostly I used it when I was away from the computer on my mobile to see if there were any useful replies to questions I had asked. It is crude, but it seems to just work. The other day I registered to be able to post. Accounts are validated manually. It took a day and a half for me to get a password to logon. I gave it a try this morning, and my post appeared immediately.
If you need another backup source for reading Usenet groups