The cutouts you're referring to are for the ambidextrous mag releases found on the 21SF and upcoming 30SF. Without a doubt these are post-ban but i've only seen them offered in .45 ACP. Unless GLOCK decides to slim down other models or release ambi mag releases for other models then it really doesn't matter.
BTW the ambi mags with the cutouts will work in the 21SF & 30SF as well as the 21 & 30but the original 21 & 30 mags won't.
The Glock 9mm 17 round mags now also have this mod. I can only imagine they will be coming out with an ambidextrous mag release on all their models in due time.