In the winter between 1999 and 2000, teams began experimenting with a flexible rear wing mounting that let the rear wing tilt back at high speed, in order to gain top speed. FIA was quick and instated a rule (that is still valid today), prescribing that the rear wing assy has to withstand a certain horizontal force (100kg) and deflect no more than 10mm iirc.
the force acting on the rear wing of course is not horizontal -downforce is around 6-8 tiems bigger than drag: so the loading condition is much different than the proof condition.
The logic explaination for a swan neck rear wing attachment to the beam wing is that it lets air flow more freely and effectively on the lower surface of the wing, which is the most important aerodianmically. But since redbull knows well how to control elastic behaviour of their wings, how long will it take before the discussion on flexy rear wing is on?