Microsoft open-sources “the earliest DOS source code discovered to date”

For students of early PC history, this isn’t even the first piece of 86-DOS history that has been newly rediscovered this ...
Ars Technica

Microsoft open-sources infamously weird, RAM-hungry MS-DOS 4.00 release

Microsoft has open-sourced another bit of computing history this week: The company teamed up with IBM to release the source code of 1988’s MS-DOS 4.00, a version better known for its unpopularity, ...

Microsoft finally open sources DOS 1.0 - and it's so much more than the code

Microsoft finally open sources DOS 1.0 - and it's so much more than the code ...

Microsoft open-sources 86-DOS 1.00, the ancestor of Windows

The source code for 86-DOS 1.00, the OS that eventually became MS-DOS and Windows, is now available on GitHub.
TechSpot

Microsoft releases MS-DOS 4.0 source code and floppy images through an open-source license

TL;DR: Microsoft will likely never release the original source code of Windows into the wild, but the company is clearly interested in sharing important episodes of its software development history.