I assume my S20+ won't get this because it's stopped getting anything but security updates. Sometime next year I'll look for the latest phone that's too old to get the new behavior.

I assume this will not be rolled out as an OS-upgrade but as a Play services update, so it will be enrolled by Google directly to nearly all devices on the market.

If so then that blows, but I'm still hopeful Samsung won't create an update for this. Unless this is something Google silently updates in the background even with automatic app updates turned off.

Samsung is not in control of this. Play services is a quite broad framework that is fully in control of Google, and the foundation for many services and applications on the device (including Play Store itself).

If you would factory-reset your device right now, it would reset to the version of Play Services that came with the installed device firmware, but upon startup the services framework would likely fetch information that it is outdated and won't continue until you have upgraded it.

In this state you could probably use your device and sideload apps, but none of the Google Mobile Services (Play Store, Gmail, Maps, YouTube,...) and 3rd party apps which require Google APIs will work