WinPE and other stripped-down unofficial "distros" of Windows do exist. Someone will try to run the Win2k shell on a Win11 kernel, if it hasn't already been done. Based on what MS has done with backwards-compatibility, I wouldn't be surprised if it almost "just works".
For compatibility mode, there's a bunch of old theming stuff. If you manage to crash a program in the right way, you can even get a Win2k-style grey-and-blue window to appear in some places, though it's hidden well.
This is the company that put the entire Windows 95 memory manager into later versions of Windows to keep compatibility, there are tons of old code paths still lying in wait for old software to reuse.
You can't use this stuff as your main UI well, it's no longer tested or optimised for that use case.
