I remember a weird misconception floating around "back in the day" that ISLisp restricted the cdr fields of cons cells to be either nil or cons. Or some sort of claim along those lines. But I see from even the 1997 draft that this is not true. I don't see anything that could be easily misread that way, either.

Where did that come from?

In fact, quite the opposite: ISLisp requires all atoms to be considered list terminators. It is documented for the length function that (length '(a b . c)) => 2.