I have 5.25 diskettes of "Microsoft Linux" from the 1990s. I'm reasonably certain that was the first.
Microsoft published its own UNIX from 1980 to 1987: https://en.wikipedia.org/wiki/Xenix?wprov=sfla1
alt Hacker News
Microsoft published its own UNIX from 1980 to 1987: https://en.wikipedia.org/wiki/Xenix?wprov=sfla1