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.
Microsoft just open-sourced 6502 BASIC (BASIC M6502 8K VER 1.1) from 1978. The code powered the Commodore PET, VIC-20, and C64, and underlies Applesoft BASIC on Apple II. Download it on GitHub to run, ...