Nitpick on the documentation of `length/2`

At

length/2

we see the length is marked ISO but is this really true?

I checked ISO/IEC 13211-1 and … nothing. Maybe in one of the corrigenda?

Its not an official ISO standard, its Prolog working group “post-N235”.
I think the working group WG17 numbered all their meeting notes.

A Prologue for Prolog (working draft)
The aim of the Prolog prologue is to avoid discussing such details
and concentrate on the identification and precise definition of these
commonly used predicates instead.
http://www.complang.tuwien.ac.at/ulrich/iso-prolog/prologue#length