Re: When did Federer lose his prime forehand?
Federer won Wimbledon outplaying Murray and Djokovic with his backhand and volleying alot in the final, his forehand was very up and down. If you attack Federer's forehand side you will get alot of points, too many players get stuck in backhand rally exchanges and dare not go to the forehand believing Federer can move to his forehand side as fast as in his peak.
Djokovic is one player who really is surprisingly passive against Federer when he got weapons to be alot more offensive, that is probably why he still gets outplayed by 31 year old Federer on faster surfaces.
All things are subject to interpretation whichever interpretation prevails at a given time is a function of power and not truth.