Interesting to hear from the pre-Tex world. I'd like to see some examples of expert use of tbl and eqn; outside of manpages the roff ecosystem seems to have vanished.
The BSDs ship with a lot of the original non-man-page Unix documentation: the system manager’s manual, the programmer’s supplementary documentation, and the user’s supplementary documentation. These are written to be typeset with troff, not printed on a teletype like the man pages can be, so they tend to be a bit more refined. https://svnweb.freebsd.org/base/head/share/doc/
They are missing some chapters (eg eqn and tbl) due to copyright disputes but you can find them in TUHS archives https://minnie.tuhs.org/cgi-bin/utree.pl?file=V7/usr/doc
roff is probably very uncommon in these days. But not unheard of. A recent example is Donovan and Kernighan’s book The Go Programming Language which was typeset using an XML and groff toolchain. It was featured on HN a few years ago, see https://news.ycombinator.com/item?id=11470905
