I would think INN2 (https://www.eyrie.org/~eagle/software/inn/) is the most widely used - it is still maintained and readily available in eg Debian.

I don't know if you would define it as "modern".

I like Gnus so much that I try to put an nntp interface on most things, but the servers that come out of that are specialized, like Feedbase or my little blog-engine, Lantern.

I never used it but the D forum has an nntp interface: https://github.com/CyberShadow/DFeed