Well, that would explain why I could no longer find Apple's trackpad settings after I noticed something was awry. At least for me, it didn't seem to have been the Windows 10 1909 update, which messed with the trackpad, though, at least not alone: after the 1909 update, it still worked fine for me, but after I got a Bootcamp driver update via Apple's own updater a week or so later, the trouble began.
However, possibly good news: after you reminded me of that, I checked the Bootcamp control panel again, and lo and behold, the "Trackpad" tab was back for me, but all multi-touch features were turned off. After activating these again, the trackpad now again works as intended.
So, I have to guess one of my reinstallation attempts restored the control panel, but I just didn't notice it because I didn't check.