The naming is a bit awkward now, but an original design goal of the Windows NT kernel was the ability to host different types of OS "personalities" (userspace, essentially) on top of the Windows kernel, and these are called "subsystems." Win32 is one such subsystem, although there was no need to formally call it "Windows Subsystem for Win32" because everyone understood that it was just Win32 ported to NT.

The NT kernel designers came from VMS, and during development MS made various half-hearted promises that NT would be able to run VMS software as well but never actually followed through. If they had, there would likely have been a Windows Subsystem for VMS.

Notably there was Windows subsystem for OS/2 though