No documentation, but you can see how @draconis was using valued_proprty_offsets and related info to do what introspection was possible on I7 properties in Programatically printing all properties associated with object?
It turns out that the property_metadata, attributed_property_offsets, and valued_property_offsets arrays don’t exist anymore in v10.
Don’t know, but I’d suspect that the first assignment is a bit of dead code that never got removed and went unnoticed 'cause it was doing no harm. (And the redundancy doesn’t exist in v10.)