> I really wish Windows 11 had a Windows 2000 mode. I want a grey, boxy UI, but I also want al the modern technologies Windows has introduced since—DirectStorage, D3D12, fast SSDs, device-independent pixels and vector UIs, all written directly against a Windows API that is modernised, safe, and easy to use. No React

I know that you said "no React" but you might want to try ReactOS. Of course if you don't need Windows-specific driver support Linux+Wine might suffice for your needs.