Is a Short Shifter Legal in the US?

Why?

A short-throw shifter or shift kit alters the lever ratio so the hand movement between gears is smaller. It does not touch the gearbox internals, the emissions system, the ECU or anything a state equipment code or federal standard regulates. It is invisible from outside the car and irrelevant at any inspection or smog check.

That makes this one of the few mods with genuinely nothing to caveat legally. There is no state where the answer differs, no paperwork, no approval and no insurance-shaped surprise beyond the general point that heavily modified cars are worth disclosing accurately.

The only consideration is quality: a cheap kit with poor bushings can develop slop or, at worst, fail to engage gears cleanly, which is a mechanical problem long before it is a legal one. A quality shifter properly installed is simply a nicer interface with the transmission you already have.