it’s a matter of extent, adding spare magazines required adding the gunmod and some support for it in the reload code. Adding magazines as an object in the game would require updating every piece of code that has anything to do with handling ammunition in the game, including large portions of the inventory code. Yes parts of this would be mechanical replacements, but it’s still a rather large undertaking.
What it boils down to is among the active devs, I seem to be the one most interested in this area, and my enthusiasm for it runs out before making it as far as supporting removable magazines as a first-order object in the game. It’s a “nice to have”, but there are a lot of “must haves” to get out of the way before working on this one.