> Where do you think that "feature" is documented?

I'd have to check, but I didn't learn this from Twitter, although it wouldn't surprise me if I learned this from a WWDC talk.

The beta for Big Sur never identified itself as 10.16 other than through this feature.  I guess Apple determined that too many applications only looked at the minor version determine if the current system is new enough. 

Applications compiled with/linked against a 11.0 or 12.0 SDK will always just see the real system version.

Note that this also affects programs just opening the SystemVersion.plist file, that will get substituted by an alternative version when the opening process links against an older SDK.  Calling sw_vers and parsing the output does return the right version though.