The title of the article is an SEO lie.

Party in the back: the brief rebirth and second death of the rumbleseat in the Sixties - Hemmings

AMC toyed with the idea with its Eric Kugler-designed 1966 AMX fiberglass show car. That then led to at least three other AMX show/concept/prototypes that incorporated the so-called Rambleseat: the 1966 steel-bodied Vignale AMX, the 1966 Smith Inland-built fiberglass prototypes, and the 1968 Jeffords AMX-R. However serious AMC's intention to produce an AMX with a Rambleseat, reality intervened in the form of the National Traffic and Motor Vehicle Safety Act, signed into law in September 1966, which paved the way for the National Highway Traffic Safety Administration and which may not have specifically forbade rumbleseats but certainly would not have treated them kindly.

Aftermarket rumbleseat kits for Cadillacs and other cars popped up here and there in the Seventies, but no automaker would touch the idea with a 100-foot pole after the National Traffic and Motor Vehicle Safety Act. So while nostalgia can indeed serve as a powerful motivator, the threat of a lawsuit seems to prove more powerful and so some trends never resurface, no matter what the time interval.