Interesting, they are removing WinRT support from .NET 5 CoreCLR and instead adding a system that lets the Windows team plug in the marshaling stuff:
https://github.com/dotnet/runtime/issues/35318
coreclr.dll is currently half a megabyte smaller than the 3.1 release.
It is based on the same ideas as C++/WinRT, which I am still bitter about, because it represents a productivity drop versus C++/CX as the ones pushing for it are hoping that ISO C++23 eventually gets the features that enables the VS tooling with C++/CX today.
So you are back to manually copying files around and writing IDL files with Notepad like experience as if using ATL style development in 2020.
Most likely C#/WinRT seems to follow the same fate in productivity drop.
The whole MDIL (Windows 8/8.1) and .NET Native (Windows 10) came from WinDev, not DevDiv, hence why it has its own idiosyncratics regarding .NET support.
Now with .NET 5, it seems they are gearing towards just a better NGEN, thus shipping a kind of AOT/JIT mixed mode image, which isn't being that well received and most likely the survey to feel the community.