Why isn't gd:visibility present in Google Calendar?

Question

According to the docs, gd:visibility should be a property on Google Calendar feeds? When I'm in Google Calendar and go to settings for a specific calendar, I copy the XML address, i.e:

http://www.google.com/calendar/feeds/myemail%40domain.com/public/basic

and change it to:

http://www.google.com/calendar/feeds/myemail%40domain.com/public/full

Then I see gd:who, gd:transparency, etc. but not gd:visibility. Have I missed something as to why this element isn't included? I need this element to differentiate which events are private and not