Actually Windows 7 do support DX 11, not 12 though, you can run UE4 games on Windows 7 so it is capable.
Windows XP definitely only supports DirectX 9, I have no idea why people still have this OS, it's a defective OS prone to errors and viruses than Windows 7.
Viewing post in How old of a PC will this run on? Windows XP?
I mean I've been in contact with a few friends of mine on the topic , one of them has Windows 7 atm and he said he already has DirectX 11 but the OS is uncapable of running it with v10.0 , our other friend diagnosed it as "old lol" , so basically Windows 7 is the main issue.
(Side note : his gpu is capable of running it but again OS limitations)