As far as I know, it is indeed a myth, sort of...

Many kitmakers are said to have decided to do so to avoid any sort of claims regarding "false advertising" or misrepresentation, but I don't believe there ever was a regulation requiring it. If there was or is, it would have affected the other model kit disciplines, so Athearn blue-boxes (model railroading) in that era on up through the yellow boxes of the far more recent offerings wouldn't have been legal, as only one example.

Several histories of the various kit-makers do state (whether its provable or not, I don't know) that the box art of the fifties and sixties was getting pushback from parents in the mid-seventies on. Vietnam didn't help at all in the US marketplace, and the mothers of the post Vietnam era thought such images weren't appropriate; the birth of the protective child-psychology era.

It rather makes sense as the children of the '60s peace & love crowd were coming of modelling age by then. I can understand the reaction by the military kit manufacturers to that, not to mention the notion that model building was becoming less widespread amongst children, while young adults (the kids of the '60s) were turning the whole thing into an adult hobby as the years marched on.

Of course, there was the time when some Israeli company, called Starfix of all things, tried to get into the aircraft kit business with a 1/48 scale Spitfire kit. It was said the only thing that fit together was the box. Lots of folks bought them and were horrified when they poured the parts out of the box; it was horrible, grossly undersized, just awful.

I actually built one to see if it was doable. It was, but only barely, and the model didn't resemble any specific mark; indeed it was a Spitfire only after a bit of squinting. There was a bit of an angry reaction to them. I might still have the dam thing around here somewhere...