At a very rough level, x86_64-pc-windows-msvc is a tier 1 target which is tested against Windows 10, but only assumes and should work for Windows 7+. There isn't really procedure for the same target triple to be different support tiers for multiple compatible OS versions, other than the footnote for Windows. It is presumed, but not tested, that the testing harness on Windows 10 would catch any issues that impact Windows 7; little/no progressive upgrading to post-Windows 7 OS APIs is done. An issue reported that only occurs on Windows 7 will be considered as a Tier 1 issue just the same as if it occurs on Windows 10.
(I knew there'd been recent discussion but I couldn't quickly find it.)
I do think Windows 7 should have been move to tier 2 long ago but the process for doing so is a bit unclear and the thread got a bit sidetracked in discussing the possibility of removing Windows 7 support entirely (as was done with older Linux and MacOS versions). The note I added was basically a compromise in an attempt to make people aware of the issue without having to decide anything immediately.
Btw, I think testing is not just an issue in CI. Firefox aside, we do seem to lack regular contributors who test nightly/beta on Windows 7 and are willing/able to send fixes when necessary. If Firefox dropped Win7 support I think we would too, at least for the standard library.