The KP_ prefix means it ought to be part of the number pad (literally Key Pad), not the regular keyboard. If there is a keyboard with such a key, I didn't turn it up with a quick Internet search. (In fact a search now turns up my mention of it in this thread via a Lemmy instance. There can't be much information about it out there if even Google is reduced to linking my comments(!))
By comparison, there's apparently a rare Brazilian number pad layout that has a dedicated comma key, which is almost certain to be KP_Comma.
I did see a couple of layouts that put a copy of the +/= key on the keypad instead of the regular +. Maybe that's what ends up with the MinPlus key code despite the name being "wrong".