However, I believe the breakpoint was 30-degrees; i.e., any gun which was capable of elevating to 30 degrees or more was given the plunging fire capability.
It's also important to remember that plunging fire was not a tactical decision on the part of a gunnery officer (i.e., No one said, "Hey-- let's do plunging fire on that target!").
Rather, plunging was the natural result of long-ranged fire... as the shells had to be lobbed higher into the air in order to cross the distance, they would come down at an oblique angle; thus the restriction that plunging fire only happens at long range.
It is theoretically possible for a gun to elevate so that its shells plunge even at point-blank range... but I don't think this was ever done...