[jamesgeck0]

You don't have to use UWP to use DX12, and you don't have to use DX12 in Windows Store games. DX12 does require WDDM 2, but that's a different bucket of worms.

Xbox Game DVR predates Game Mode; you can enable/disable them independently.

[WorldMaker]

Also, you don't even have to use the Windows Store to install UWP games. Windows 10 allows sideloading by default (since the November Update two years ago, though shortly after Valve made their stink) and since the Anniversary Update (1611) last year this time, there is a nice little installer that pops up if you double click a (signed) UWP APPX package.

So even if DX12 was restricted to UWP apps (which, reminder, it isn't) UWP apps are not restricted to the Store either, and Steam could install UWP apps, too.