There's no official driver support for Windows 7/8.1 but other than this there was nothing to prevent you from installing an older version of Windows on a Ryzen or Kaby Lake machine. This is no longer the case as Microsoft is getting more aggressive with its push to upgrade users with modern processor architectures to the latest version of Windows.
Since this month's Patch Tuesday, users running Windows 7/8.1 on a PC with the Intel Kaby Lake or the AMD Bristol Ridge or Summit Ridge based processors can no longer use the Windows Update feature. Microsoft recommends users to upgrade to Windows 10 because this is the only version that supports the latest processor architectures.
When you try to scan or download updates through Windows Update, you receive the following error message:
Your PC uses a processor that isn’t supported on this version of Windows and you won’t receive updates.
Additionally, you may see an error message on the Windows Update window that resembles the following:
Windows could not search for new updates
An error occurred while checking for new updates for your computer.
Code 80240037 Windows Update encountered an unknown error.
This error occurs because new processor generations require the latest Windows version for support. For example, Windows 10 is the only Windows version that is supported on the following processor generations:
Intel seventh (7th)-generation processors AMD “Bristol Ridge” Qualcomm “8996"
Because of how this support policy is implemented, Windows 8.1 and Windows 7 devices that have a seventh generation or a later generation processor may no longer be able to scan or download updates through Windows Update or Microsoft Update.
We recommend that you upgrade Windows 8.1-based and Window 7-based computers to Windows 10 if those computers have a processor that is from any of the following generations:
Intel seventh (7th)-generation "Intel Core" processor or a later generation AMD seventh (7th)-generation (“Bristol Ridge") processor or a later generation Qualcomm “8996" processor or a later generation