Yeah there is no problem adding a toggle to turn rage mode on and off for all difficulties.
Having rage mode turned off would actually be less CPU intensive as there is no need to check if the zombie gets into rage mode or not. You seem to forget that there is already a way to play with no rage mode in vanilla on the lowest difficulty and that doesn´t impact performance at all.
Still no real reason why you are so against it. Unless you are a dev in disguise. And even then, people ask for things all the time and that doesn´t harm the devs at all. Not a single bit. They can simply ignore it as they do anyways most of the time. So nothing would change if they add a toggle for rage mode.