OSHA does not have a requirement that forklift vehicles be equipped with a backup alarm or warning light (although these features are standard on most new equipment).
A forklift must have, at minimum, some type of sound-producing device that an operator can use at cross aisles and blind intersections where vision is obstructed to warn pedestrians and other forklift operators that they are approaching. The “sound-producing device” mentioned in the Powered Industrial Truck Standard is a horn.