I'm curious as to where you saw that. While I've heard of such requirements for big rigs and box trucks, I've never heard of any US State requiring mud flaps on a pickup truck. The little plastic "spats" on the RAM trucks cover the requirements some states have for "splash preventative devices".
I look up driving laws for states and provinces I travel to. I search for the odd stuff that might trip me up like fines for fog light use, honking, not locking a vehicle, or having a dash cam. I also look for the penalties, and possible issues if stopped.
In addition every now and then a mass enforcement event or person who just can't believe there is such a rule escapes the local news bubble, or I'll be cruising local news and catch a discussion, which will prompt me to look up that areas driving laws too.
Washington make it straight forward, you need fenders or flaps as wide as the tires behind which they are mounted and that extend downward past the center of the axle.
https://apps.leg.wa.gov/rcw/default.aspx?cite=46.37.500 . California only specifies width in their law, but has officers give tickets and warnings based on heights not listed in Vehicle Code 27600. Others make it over complicated with math or items that are discretionary.
There are a few sites like this one that have summaries, but they often have incorrect or out of date information, so I recommend a quick google search for mudflap and the state to verify.
https://www.grdproductsco.com/blogs...-flap-fender-requirement-laws-you-should-know
I've not come across a state where a spat would make a difference, but I have seen one that limits the size of your fender flares.
