In modern x86 CPUs, POPCNT is implemented as part of the SSE4 instruction set. For Intel's chips, it was added as part of SSE4.2 in the original first-generation Core architecture, codenamed Nehalem. In AMD's processors, it's included in SSE4a, first used in Phenom, Athlon, and Sempron CPUs based on the K10 architecture. These architectures date back to 2008 and 2007, respectively.

That effectively bars mid-2000s Intel Core 2 Duo systems and early Athlon 64-era PCs from booting Windows 11 at all, not that they officially supported it in the first place. This means the change should mainly affect retro-computing enthusiasts who spend their days making YouTube videos in the "we installed Windows 11 on a potato, let's see how it runs" genre rather than users of actual systems.

You can check your SSE support for 4.2 and 4a respectively but it sounds like unless you're running some real old stuff you shouldn't have to worry.