https://en.wikipedia.org/wiki/Workplace_OS
The 1990s were quite a time for personal and workstation computing.
I think the OS/2 subsystem was 16-bit OS/2 1.x so nobody cared and the POSIX subsystem was just compliant enough to win government contracts.
This design is why we have the "Windows Subsystem for Linux" (a name everybody hates) because "Windows Subsystems" were already a thing in Windows.
Docker, Distrobox, and even Flatpak are one kernel with multiple "personalities" but they are all still Linux I guess.
You can also argue have this on our desktops today with things like KVM in Linux and Hyper-V in Windows.
I also bet that many others would not have cared, and used UNIX/NT personality if that was the case.
But every subsystem other than Win32 was kneecapped mostly due to politics and market positioning.
In late 90s Microsoft bought a company which had developed a more enhanced Unix subsystem and rebranded it as Interix and marketed as Windows Subsytem for Unix (SFU).
I believe the original WSL was a resurrection of SFU before WSL2 pivoted to a VM-based approach.
IIRC, Interix still used same approach as original posix subsystem (and Windows and OS/2 subsystems) of providing the interface as DLL that ultimately your application would be linked against.
https://www.microsoft.com/en-us/research/project/drawbridge/
https://learn.microsoft.com/en-us/archive/blogs/wsl/pico-pro...