Blade Below the Shoulder: Some types I and II exist; the former because a strap-on “weapon pod” is a standard accessory for quick-draw weapons, some of which might as well be blades, and the latter because – well, while they’re as impractical as concealed weapons as any other cybernetic implant, X-rays being a thing, and probably not the most useful thing you could stash inside your arm anyway, some people have funny tastes and like the notion of weapons attached to them.

Type III is certainly possible, but it not being considered an idiotic self-handicap probably isn’t. That discourages people.