Thus is quite interesting IMO, the fact that it will work on apparently all devices with no version limit will probably be a surprise to many.

I suspect, given the fact that ADB can bypass the requirement, that Google is using the pre-existing Android mechanism to gatekeep app installs which is currently used for Google Play Protect. The Advanced Protection variant of this removes the "Install anyway" option [1] which is quite irritating, either you have to install every new F-Droid apps from ADB or disable Advanced Protection which I ended up doing.

(I wonder if Shizuku can be used to install apps without a PC? Haven't tried myself)

[1] https://support.google.com/accounts/answer/9764949?hl=en