Hm, looking to see if the vendor field is actually meaningful ... I see some stuff for m68k and mips and sysv targets ... some of it working around pre-standard vendor C implementations

Ah, I found a modern one:

  i[3456789]86-w64-mingw* does not use winsup
  i[3456789]86-*-mingw* with other vendors does use winsup

There are probably more; this is embedded in all sorts of random configure scripts and it is very not-greppable.