- Everything but Linux and a few of the more popular BSDs
- 32 bit CPUs
- CPUs with less than 1,000 installations
- Ancient toolsets that don't support features everyone else takes for granted in the last 20 years
For instance, it has to be a pain in the neck to sit on a code base with Solaris 2.6 support, or NetBSD on Alpha, or... Also, it seems like there'd have to be some legacy pain around things like "we can't use this flag on sed because Amix didn't have it".
But I don't know enough about the infrastructure to know if pruning those out would make a bit of engineering difference. How much is support for ancient or little-used stuff slowing down development?
Edit:
Alternatively, I wonder what it would look like to have a build farm that precomputed all the values. "Oh, you're on macOS 11.1 on Intel? Here's the list of 32 envvars you need to set." If a million people are compiling the same file on a million identical computers, is it a great idea for all of them to have run the same probes?
Edit 2:
I'd never advocate for support for such systems to be scrubbed from the Internet. I'm just saying it's unreasonable to expect a maintainer today to support ancient systems. The people still using those systems have the right to fork the tools and maintain their own version, but that's not the same as making upstream do it for them.