Hacker News Reader

This is one of many free replacements for google groups. I have been using that for years.

Not exactly it does newsgroups only which is a small part of Goole groups.

Most of Google Groups is mailing lists which can just be created unlike a newsgroup.

Check if your country has public-funded LISTSERV/Mailman/* instances. If not, I'll check https://gaggle.email

Checkout Topicbox by Fastmail... fills the niche perfectly!

Is it for a mailing list or for a website archive? You can self-host a listserv on many shared hosting sites for pennies.

What are the requirements / key features of Google Groups that you use?