Code violation? Those are strong words!

Gfretwell is correct. Indeed, there is no requirement that anything plugged into that receptacle be listed or code compliant.. The NEC stops at the breaker. FWIW, the RV plug pattern isn’t found on the NEMA chart.

Apart for use feeding a smaller breaker — and, no, coordination is not required — another possible legitimate use of such an adapter might be in feeding a welder.