"where is this documented ?" in the materials related to that Taco class action lawsuit.
The shape of the frame isn't significant, it is the way the members are welded to each other.
A thin sheet metal body of a car (and of our FJ body) would rust away very, very quickly, but the edges of the panels are all covered and protected so that there is a uniform coating over all of it, blocking exposure to water and oxygen. Also, the spot welds are not susceptible to starting rust.
A truck frame is quite different from that, so even though it is many, many times thicker than the thin sheet metal body, the edges of it rusts rather quickly. But, because it is thick, the industry has not cared much about it.
Look underneath any OEM truck, and see how most are covered with red rust after just a few seasons (in the rust belt).
The issues with the truck frame are:
- the stick welds rust quickly, because the factory coating cannot bond to them as well as to the rest of the surface
- the thick steel frame members end with sharp edges, which result in thin/no coating on each edge, so rust can start.
The steel is quite thick, so, historically the red rust has been considered merely cosmetic. For most OEMs, a 10 year life is doing pretty good, so perforation was a pretty low risk for them. But, for Toyota, with these days typically >20 year life span, that rust becomes a bigger issue over time, leading us to where we are now.
The taco lawsuit was because the coating was incorrectly applied, so they had areas of bare steel, which rusted through much faster than 10 years.
N