Oh, come on. You don't need a 3D printer for this. All you need is a piece of sheet metal and a CNC milling machine (or heck, even a manual milling machine and some skill at using it), that every moderately equipped machine shop has on hand.
You can't prevent people from having tools that can be used to manufacture things, and no one is seriously going to try.
Goodness, you don't need a milling machine, you can probably do this work with a dremel, or for that matter a file... You know, like locksmiths made keys decades ago.
Actually, 3d printer is a tool that's currently cheaper and more widely available than milling machines - the whole point of this is "hey, of course anyone with access to CNC or the skill to operate a manual milling machine could do it, but that's only a small niche, but now every idiot with access to a 3d printer can do it".
People and shops that are below the range of a moderately equipped machine shop can do 3d printing; a 3d printer that can make such a key from hard plastic costs less as a cheap manual milling machine that can make a similar key from metal and requires less skill to operate.
That would restrict it to locksmith basically (professional or hobbyist), as opposed to anyone that can find out how to run a 3D printer. Being a locksmith isn't scalable, using a 3D printer is.
No, if you can make a 3D printed model of it, you can CNC mill it. Sure, a CNC milling machine is a bit more expensive than the cheapest 3D printers, but anyone who wants can find a machine shop nearby that will mill something for them, or just send it off to some place online like eMachineShop to get it done.
and actually, it's not even that much more expensive. You can get a CNC Sherline mill for something like a $1500. Sure - the work area is small, they're mainly aimed at e.g. model builders, but it's big enough for something like this, and they're real honest to goodness CNC precision machine centers.
I fully expect we'll see some attempt at regulation at some point because the answer to "you wouldn't download a car, would you?" is actually an emphatic yes. That is to say, when the ability to turn information into product becomes easy and decentralized it will absolutely threaten any illusion governments have of control through standardization. Regulations are the clear path, for better or worse...
I fear, some governments may seriously have an idea of restricting 3D printers.