Mud flaps are only required on the rear set of tires, and they are only required to come down half the length (or diameter) of the tire.
When I drove a truck, I was regularly subjected to routine DOT inspections. If you see mudflaps that worn, they must be on local trucks, but as long as they come down half way, they are legal. Many mud flaps start out longer than they need to be.
It is true that the police don’t think this is a high priority, but most states have dedicated DOT officers to enforce these laws.