The 223 will kill deer with a proper bullet properly placed. But its the bare minimum of a round.
There are many "less powerful" rounds that will also kill deer with a proper bullet, properly placed. And, like the .223, some are legal to use in some places and not in others. There are even places where some less powerful rounds are legal and the .223 is not. And, there are also some places where ARs (or other semi autos) are not legal for deer. And some places where no rifle is legal for deer but shotguns are.
Looking at my personal collection of .22 centerfires, my .223 would be my last choice for deer. If I had to, I would use one of my .22 Hornets, or .222s or even my .221, and certainly consider my .22-250 over my .223.
Because of the guns they are in. And by that I mean their accuracy and weight. I have .22 Hornet and .222 Remington in rifle and pistol, my .221 is a pistol, and my .22-250 is a varmint rifle. All are scoped, and capable of the precision needed for a head or neck shot within the cartridges efficient range.
My .223 is not. Its an older Ruger Mini 14 with iron sights, and not nearly as accurate as my other .22s. Which is a moot point, anyway since my state has a minimum .24 caliber requirement to be legal for deer.
I have never been a fan of any of the AR rifles for hunting. Not even the larger calibers. I hunt deer (and other big game) on foot, for sport and happily trade firepower (magazine capacity) for weight reduction. Even when made of wood and steel a bolt gun, a single shot, and several lever guns can weigh less in the same calibers as an AR pattern gun, and often has as much or more barrel length as well, if I choose that.
It's a personal choice, and I won't fault anyone who hunts with an AR type rifle, if that is their personal preference. Its just not my preference.