AMD Ryzen will have official drivers for Windows 7
In a stark contrast to Intel whom is no longer providing official Windows 7 drivers for its current generation of Kaby Lake processors, AMD has made it very clear that it will develop and release appropriate Ryzen drivers to be as compatible as possible with the outgoing Windows OS.
Despite AMD's extended commitment, the drivers may never be perfect or fully featured as Microsoft ended mainstream support for Windows 7 over two years ago. The Redmond company will only continue to release security patches and bug fixes that will not likely affect Ryzen compatibility in a direct manner. Certain features of Ryzen could therefore never be compatible with systems running Windows 7 compared to systems running Windows 10.
This isn't to say that Kaby Lake cannot run on Windows 7 at all, but official support from both Intel and Microsoft is essentially nonexistent. Even if Microsoft shuns Ryzen users running Windows 7, AMD will still be putting in effort for better compatibility at least for the time being.