We just open-sourced DOS 4 (and found binaries of Multitasking DOS 4) https://www.hanselman.com/blog/open-sourcing-dos-4
Open Sourcing DOS 4

See the canonical version of this blog post at the Microsoft Open Source Blog! ...

@shanselman Under an OSI license, or one of MS's not actually open source licenses?

@BoydStephenSmithJr @shanselman "All files within this repo are released under the MIT License as per the LICENSE file stored in the root of this repo."

Straight from the GitHub repo

@me @BoydStephenSmithJr I own the OSPO, Boyd. We’ve done MIT for most everything for over a decade or more.