We knew Microsoft was planning to block installation of Windows 7 and 8.1 updates on systems with Intel 7th Generation Core processors (more memorably known as Kaby Lake) and AMD Ryzen systems; we just weren't sure when. Now, the answer appears to be "this month." Users of new processors running old versions of Windows are reporting that their updates are being blocked. The block means that systems using these processors are no longer receiving security updates.
The new policy was announced in January of last year and revised slightly a couple of months later: Kaby Lake and Ryzen processors, and all new processors on an ongoing basis, would only be supported in Windows 10. Windows 7 and 8.1 would continue to support older processors, but their chip compatibility was frozen.
Awkwardly straddling the two policies are Intel's 6th Generation Core processors, aka Skylake. Some Skylake systems will continue to be supported in Windows 7 and 8.1. Others will not. Certain Skylake models shipped by 16 specific OEMs will continue to receive update support. But other Skylake systems will also need to upgrade to Windows 10 to receive ongoing updates.