On the topic of whether old GCCs are "useful":
Based on past discussion, I believe it was decided that in NetBSD's case,
a verison of GCC identical to base should be used to provide a Fortran
compiler. This would mean keeping at least GCC 7 around for NetBSD 9.
A while ago, I removed gcc48/gcc49/gcc5 becaue I determined them
non-useful for bootstrapping because GCC versions as high as 10 can be
built with only a c++98 compiler (same as versions as low as gcc48).
Additionally, these compilers weren't useful on RHEL derivatives
because they couldn't be built with FORTIFY_SOURCE, which in many
cases we are enforcing by default in pkgsrc.
I would argue that gcc6 should go because it's no longer useful.
GCC 8 and GCC 9 could also go from the infrastructure now.
I'd also argue that for consistency's sake, and for reasons
of better testing and reproducibility, it makes sense to primarily
focus on providing GCC versions that were included in a NetBSD
release, as well as more recent versions.