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/
Even if dx12 is an arbitrary restriction to only work in w10 that’s beside the point. It’s a feature of win10 no matter how arbitrary.
There was no need for apps to ask that. Previously, apps would just say "draw this dialog box in the user's preferred color scheme" and it would work fine. The only reason this dark mode hint is necessary is because too many apps started ignoring the Windows system color scheme and doing their own thing.
The difference to windows users is that you change a switch and apps actually change whereas before you couldn’t do that.
It wasn’t Microsoft’s fault before and it isn’t they who updated the apps now so they don’t get credit for that. But the fact remains you basically couldn’t use dark mode before and now you can.