Is there a subset of this group for people who like to
program in
languages or language implementations or libraries that are no longer
in common mainstream use? Or other groups for such a thing?
I don't think of it as "retrocomputing" per se, but I maintain a
SNOBOL4 implementation:
http://www.snobol4.org/csnobol4/
Mostly I write SNOBOL4 throw away programs for textual
transformations. The biggest bit of programming I've done recently is
a package to compile/install shared libraries for extending SNOBOL4.
Phil