[@role model]

The big issue with implanting anything in the head is that that's basically brain surgery, which requires specialised staff and is risky to do. In the wrist (or somewhere around there, in the lower bit of your arm) is not only a lot less risky, but also requires less specialised training and therefore is several orders of magnitude easier to do on large scale. People avoiding ID printing is an issue at any point of course since in this kinda setting births will still occur outwith of hospitals, but most folks will have a run in with the law or with some sort of 'social service' at some point in a dystopia so they can just get tagged then if they haven't been otherwise - a little bit like how Han Solo only gained the surname Solo when he registered with the Imperial Flight Academy or whatever; until that point he'd only been Han and hadn't legally existed, past that point he was a mark in a book in the Imperial records office even if he didn't get tattooed and chipped. With a wrist chip a police medic could probably get the training to stick one in, using local anaesthetic (if any).