I'm betting more on "replaced". E.g. When they switched to Webkit, they didn't opensource Trident. How many people would notice, if the next version of windows used CUPS running on the WSL2 backend with a nice small GUI in Windows-Land? The very informative comments from @zinekeller and @ChrisSD highlight the problems with opensourcing mixed-vendor heterogeneous code bases. Also open-sourcing code rarely means it becomes "maintenance-free".

Psst! They technically released the ECMAScript/JScript (definitely not JavaScript®)* interpreter, Chakra (https://github.com/chakra-core/ChakraCore).

* Trademark notice: JavaScript® is a registered trademark of Oracle America Inc.

I think you're right in that the road to MS open-sourcing Windows will be slowly starting to adopt other existing subsystems (and hopefully giving back) so that they are managing less and less proprietary code over time.

WebKit? Edge is using Blink, unless I’m misunderstanding?

Blink is the name they gave the fork of the webkit core.

The fork was years ago at this point. Blink and Webkit certainly share a lot of code, but I think it's inappropriate to completely lump them together.