I configured Qt for iOS without -skip qtpurchasing but with -skip qtandroidextras, and then noticed that qtpurchasing module wasn't actually compiled. Afterwards I checked .gitmodules and noticed that it depends on qtandroidextras which makes no sense for non-android builds:

[submodule "qtpurchasing"]
	depends = qtbase qtandroidextras

Next time I didn't add -skip qtandroidextras and qtpurchasing appeared.

I guess it's the same with macOS build.