I was just a kid during the 1990s when all of this was happening, but a few years ago I remember reading about an IBM project named GUTS where one kernel would run multiple OS "personalities":
Microsoft technically delivered something very close to OS/2’s “Personalities” in Windows NT 4. They called it "Environment subsystems". Each subsystem could run applications written for different operating systems, the 3 available ones were Win32, OS/2 and POSIX. Then there was the "Integral subsystem", which operated system-specific functions on behalf of environment subsystems.
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.
No, the original WSL was a weird new thing where NT kernel-level driver actually serviced Linux system calls.
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://en.wikipedia.org/wiki/Workplace_OS
The 1990s were quite a time for personal and workstation computing.