For technical discussion groups: USENET. Either moderated newsgroups or even unmoderated ones for obscure enough topics. Spam is the issue, but not much spam on USENET these days.

Also, the ham radio guys like groups.io, which is where yahoo groups migrated to. There are a lot of groups there on specific things, like Yeasu FT-857D transceiver. Groups.io has per-group file storage, which is handy for this.

Anyway on USENET, you can access it via google groups, but it's a bit non-obvious. Here is an example group:

https://groups.google.com/g/sci.math

But how can you find groups? You click on "all groups and messages" on the top, then enter a search term and hit enter. Then click "outside my org". When I do this for the term "france" it shows "4635 groups outside your organization". Open the list and look for the groups that have "." separators, for example "alt.france".

https://groups.google.com/g/alt.france

The ones without the dot separators are google groups- if you post, the message does not go outside of google. I don't know if you can create a USENET newsgroup through google groups- probably not.

It's kind of crap for discovery, but OK once you have a set of groups you use often.

Edit: actually a better way for discovery is a website like this: http://www.harley.com/usenet/master-list/index.html Once you have the name of the newsgroup, just append it to the groups.google.com/g/ (also the site provides a link).