https://wccftech.com/microsoft-open-sources-ms-dos/

Microsoft is making the source code of MS-DOS available to the public after over 30 years. If this sounds like deja vu, the Redmond software maker did offer the source code of MS-DOS as part of a collaboration with the Computer History Museum back in 2014. The 1983 operating system was added to the Computer History Museum in 2013. However, the latest move is part of a “re-release” without the restrictions of the previous effort.

The new repo that was made available on GitHub contains the original source code and compiled binaries for MS-DOS 1.25 and MS-DOS 2.0. All the source files are available for reference and internal experimentation as the company isn’t allowing pull requests or modifications to the source.