Gmane was relatively popular, and it was great but went down for a long time, which probably further obscured NNTP. IDK if everyone who used to use it is aware that it's up again, now at http://gmane.io/
Thunderbird works on most platforms.
> Gmane was relatively popular, and it was great but went down for a long time
Some mailing lists are now archived using public inbox[1]. There are several ways to access it including via NNTP.
Can you get your own mailing lists on public-inbox, or would you have to self host? I'd love to self host git and a mailing list but setting up mailman/sympa/etc. seems to be too big a task. I was thinking setting up a wiki as a replacement for my small personal projects...