Oracle's SunOS (now Solaris), HP-UX and IBM's AIX, plus the community-supported *BSDs and Darwin would be the only ones you'd see in the wild today, from that list. IRIX and Tru64 are retired or history, according to Wikipedia. ZDNet in April 2014 reports that Unix server share as a whole is 13.6% according to IDC, Oracle has 4.7% leaving 8.9% to competitors. Since many of these are RISC or Itanium servers, leaving them out means fewer compatibility headaches on multiple fronts. That all said, unless there's a testing service out there for this kind of enterprise hardware, I can't see how anyone writing open source code outside of large enterprises would be expected to support this configuration without patches from the community... which is probably why "standard checks" remain today. Nobody really knows if they're still needed or not, since the original patch might go back to the mid-90s.

Yeah, that may be true today, but it certainly wasn't when these tools were conceived. (According to wikipedia, autoconf was first released in 1991.)

But you don't have to go on using it...