A while back, Microsoft let the world know that it'd only support recent CPUs in the Intel Kaby Lake, AMD Ryzen, and Qualcomm 8996 families under Windows 10. Now, there's a little smoke and fire in some corners of the internet, though, since a Microsoft Knowledge Base page has popped up about a potential error message that users with systems based on the forementioned CPUs might encounter when running Windows Update. More to the point, you might be told that "your PC uses a processor that isn't supported on this version of Windows," and that you'll need to upgrade to Windows 10.
Despite this bit of news making the rounds around the web, and the fact that there's a running Reddit thread with close to 600 comments on the subject, there have yet to be reports of systems in the wild where this error message popped up. The support page doesn't appear to be a fluke, though, since searching the Knowledge Base for article 4012982 yields results in multiple languages.
The company stated back as early as August 2016 that Windows 10 is the only supported version for new-generation systems, and its FAQ says as much:
As new silicon generations are introduced, they will require the latest Windows platform at that time for support. This enables us to focus on deep integration between Windows and the silicon, while maintaining maximum reliability and compatibility with previous generations of platform and silicon. For example, Windows 10 will be the only supported Windows platform on Intel's upcoming "Kaby Lake" silicon, Qualcomm’s upcoming "8996" silicon, and AMD's upcoming "Bristol Ridge" silicon.
Rather predictably, some commenters are up in arms about this event, assuming it ever pops up in production systems. However, as recently as January, Youtuber JayzTwoCents installed Windows 8.1 on a Kaby Lake Core i5 system without fuss. Veering a little into an editorial piece, there are a few things to consider. The whole situation begets the question of what are the expectations of users trying to run what's effectively a legacy OS on their new boxes, since they already knew what the support policy is. Ma and Pa may not, but it's not likely they're installing a fresh operating system on their computers anyway.
There's also the matter of install media to consider. While there's a Media Creation Toolkit for Windows 10, most people trying to install Windows 7 or 8.1 will use a DVD or USB stick that (as far as we know) doesn't appear to have a CPU generation check, and possibly never will. That means that in order to not let people use an unsupported OS, it makes at least some sense that the check be done in Windows Update.
Technically-minded users might point out that they're fine with running an "unsupported" operating system at their own risk, but the average user is likely not even aware of Microsoft's policy regarding Windows 7 and 8.1. In turn, he may encounter problems that he doesn't know the origin of, much less how to fix them.
Flipping the subject on its head, it could be argued that at least Windows 8.1 could be supported on those CPUs, since its support ends in 2023, while Windows 7's EOL is reasonably close in 2020. At this point, it's difficult to tell if Microsoft's policy and execution on the matter creates a real problem.