Dark mode being use as a short hand - pretty much all "standard" controls used to have colors and font size defined. So if an application wants to draw text - it'd use the text area background and color, likewise for buttons. Being replaced with a single boolean configuration option is just a lazy downgrade. Also I don't quite see it as an OS function - in the end it just reads the registry.
Vulcan was supported on Win7 (along w/ the raytracing) and oddly enough Win7 had a port of DX12 by Microsoft [0]. It was quite an arbitrary decision to prevent Win7 & 8 to run DX12. I suppose one of the issues is that GPU drivers (esp. AMD) do not support Win7 (or 8)
[0]: https://venturebeat.com/pc-gaming/directx-12-windows-7/