Are Drilled and Slotted Brake Discs Legal in the US?

Why?

Brake rotors are treated like any other wear part: no federal standard dictates whether the friction surface is plain, drilled, slotted or both, and plenty of cars leave the factory with drilled or slotted rotors already. State equipment law requires an effective service brake system, and a healthy set of performance rotors satisfies that exactly as the originals did.

The considerations are engineering, not legality. Cross-drilled rotors shed water and gas well but cheap ones can develop cracks around the holes under repeated hard heat cycles, which an inspection state would fail as a defect, the same as any cracked rotor. Slotted designs trade a little pad life for consistent bite and are more tolerant of abuse.

Buy from a reputable maker, match the rotor to the use, and there is nothing further to it: no certification, no declaration, no state where the answer differs.