If it ever was an option (I don't remember that it's ever been one), it shouldn't be. What would be the reason for wanting to lower the charge rate? The only reason I can think of is that you want to dwell longer at the Supercharger (for whatever reason), but if you think about it, this goes against the point of Supercharging and unnecessarily ties up a valuable resource. So, the rule is that it charges as fast as possible.