The WSL2 GUI application initial preview release is finally out, with source code implementing two new RDP virtual channel extensions in FreeRDP. While the source code can be used as reference for the RDP protocol changes, it would be a whole lot better to take a look at the corresponding preview specifications. Would it be possible to publish them in the Preview Documents section?
There is a new "gfxredir" virtual channel extension with dynamic virtual channel name "Microsoft::Windows::RDS::RemoteAppGraphicsRedirection" that is an improved version of RemoteApp using shared memory, and adapted to remoting Wayland applications.
There is a new "rdpapplist" virtual channel extension with dynamic virtual channel name "Microsoft::Windows::RDS::RemoteApplicationList" that is used to facilitate listing and launching applications available in WSL2 from the Windows host.
I haven't checked what kind of modifications have been done to the existing specifications aside from these two new virtual channel extensions, but I wouldn't be surprised if the older RAIL extension has been updated.