A lot of things determine whether a given trailer will need WD or not. If the tongue weight is above 10% of the GVW of the trailer, and the receiver of the tow vehicle is capable of carrying that weight, and the front suspension of the tow vehicle is not affected enough to affect the handling of the rig, and the ride and handling are satisfying to the owner, then I would say WD is not needed.
However, if any of the above stipulations is not met, then I would say WD is needed. Most travel trailers of any length will require it because their tongue weight will exceed the weight carrying rating of the tow vehicles receiver. Many modern trucks have receivers capable of carrying much higher tongue weighs than used to be the case. That fact does not keep the front axle of the truck from being unloaded however and possibly affecting the handling. The squat of the truck does not determine whether WD is needed or not. It may be a good indicator though. The WD hitch can help with that condition as can air bags. Air bags are not a substitute for the use of a WD however.
In addition, most owners use them because of the sway control most modern hitches incorporate into their design. Most owners have found that the ride and handling of their rigs have improved because of the use of the WD hitch.
As Ben mentioned above, there are many different variables that affect whether a WD hitch is needed or not. It is up to the individual owner if they want to use one or not. There is no law that determines this - at least that I know of.
Hope this helps a bit.
๐Barney
2004 Sunnybrook Titan 30FKS TT
Hensley "Arrow" 1400# hitch (Sold)
Not towing now.
Former tow vehicles were 2016 Ram 2500 CTD, 2002 Ford F250, 7.3 PSD, 1997 Ram 2500 5.9 gas engine