> If Win8-only calls are not used, then presumably it should still build and run on Windows 7, presumably with the flag flipped back to Win7. And if there are Win8-only calls used and the flag is set to Win7+, I assume that the MSVC compiler will catch any instances of Win8-only calls at build time? Is this understanding correct?

It's the latter. In the patch, calls to functions like PathCchCanonicalizeEx(), which had previously been conditional on runtime availability of those functions, were made direct. Those functions are not from UCRT (so you can't just distribute it to regain Windows 7 support) and are not available in Windows 7.

There was also another patch that adds a check to Python 3.9+ installer that prevents installation on Windows 7 (but again, even if you take the Windows "embedded" distro instead of the installer, it won't run due to Win8-only functions).

> If the latter is true, its very likely a lost cause. However, if the former is the case, then at least devs using Python 3.9+ on Windows 7 will be able to easily build their own versions with support, though that could change at any time.

For now, it's not entirely lost cause in the sense that the number of Win8-specific patches and their size is very small (I'm not aware of anything not already mentioned, but I may be not up-to-date). So an interested party can probably revert them locally without much hassle.

> For those in the know, have there been a lot of reports/discontent surrounding the dropping of support in Py3.9, by anyone aware of that? 

I semi-casually follow the bug tracker and remember only bpo-41412 and bpo-42027.

Personally, I do consider tying of Windows support in CPython to arbitrary support periods set by Microsoft unfortunate, and it's also true that Win8-specific changes I'm aware of are not critical and don't remove any significant maintenance burden. For now, both relative and absolute usage of Windows 7 is huge and hard to ignore, so developers who use CPython in their own software are faced with an unpleasant choice of sticking to 3.8 or dropping Win 7. (I'm one of them since software I work on has to run in build environments setup for other projects, and such environments routinely stick to old OS versions due to breakage on updates). But this is what we have according to PEP 11, and a little grumbling here certainly won't change that :)