Border vs BorderOpacity

When I look in the debugger, Graphics like OvalShape, RectShape, CurveShape etc. show a Border property. It seems to track with the BorderOpacity. You can assign it a value in code and it seems to behave as if you have assigned the property BorderOpacity that same value.

And yet in the Documentation, I cannot see Border listed as a Property of these Object2Ds. Is there a reason for this? Is this a relic from former times?