One of the most challenging things Microsoft has faced when open-sourcing projects is the auditing stage. Windows is huge. Lots of code in Windows are not written by Microsoft or even by third-parties contracted by Microsoft. Some parts of Win32 code (that is backwards-compatible sensitive) are owned by third parties that are already long gone or absorbed by other third parties. This is the reason Microsoft cannot even release a binary version of Space Cadet Pinball even when they want to because it is now owned by EA (Surprise! Also see https://devblogs.microsoft.com/oldnewthing/20181221-00/?p=10...) and the same reason why they have removed the pre-2007 equation editor on their Office suite (https://support.microsoft.com/office/equation-editor-6eac7d7...). At one of its previous audits (for IE 7), they have to check if it was Spyglass code and replace the offending code which allows them to terminate their contract with (at the time) AOL.

Windows has many of these components, and they are even attributed when you know where to look. For example, the code for parts of the disk management utility, the spinning-disk defragmenter, and NTFS quota management were based from code provided by VERITAS Systems (which I am not even sure if the company still exists). The MP3 codec is provided by Fraunhofer (which still exists, but I'm sure that they will not agree to open-sourcing that codec). On the other hand, some of them are under permissive licenses or even the same code as other counterparts (while the BSD TCP/IP stack story ranges from code removed by Windows 2000 to simply an apocryphal tale) for example, the code by IJG for JPEG support is used extensively in Windows. The only built-in (L)GPL code used ever was the BRLTTY system (https://mielke.cc/brltty/index.html) and LibLouis (http://liblouis.org/), which was used for Braille accessibility (WSL2 used the Linux kernel, but they are arguably a different application with separate instances).

Edit: Microsoft's third-party disclosures: https://www.microsoft.com/en-us/legal/products/notices/win