Not only legal but strongly encouraged by our instructors to prevent hearing loss. There are riding specific earplugs tuned to block the wind and road noise more than sirens, horns, etc
It might surprise you to see how many states it is illegal in (IIRC it's close to half). This may even include states in which the motorcycle school instructors tell you to wear earplugs for hearing loss.
That isn't the case. The majority of the states have no law and a good chunk of the others have exceptions for hearing protection. A lot of the websites online have outdated info- e.g. most websites claim CA and OH ban earplugs, when this clearly isn't the case.
The California carve-out for hearing protection states: "The plugs or molds shall be designed in a manner so as to not inhibit the wearer's ability to hear a siren or horn from an emergency vehicle or a horn from another motor vehicle."
Ohio has a carve out specifically for motorcycles; operators of vehicles cannot (I used to own an RV where the wind and road noise at freeway speeds was well above safe levels for sustained operation).