DirectX 12 will be there in Windows 10 and will be compatible with Windows 8 but it won't be supported by Windows 7. AMD's Chief Gaming Scientist Richard Huddy said in a PDXLAN presentation that Windows 7 remains Microsoft's most popular operating system with 52 percent of Windows userbase. But in spite of that, "One thing that's not going to happen to it is DX12. Yup, DX12 is not coming to Windows 7."
According to Steam hardware survey, 47.8 percent of users run Windows 7 64-bit version, compared to 21.4 percent for the second-place finisher, Windows 8.1 64-bit. Let us also not forget that Microsoft announced this past spring that mainstream support for Windows 7 will come to an end in January 2015.
As you know, DirectX 12 is a huge update! It promises increased performance in games, thanks to reduced CPU overhead and better support for multithreading, and significantly reduced power consumption for gaming laptops.
Via PC Gamer