In #12159, Victor correctly labelled this an implementation (IE, CPython) detail (limitation). I don't believe any implementation has to limit the range of len(). So the question is whether we should add a CPython implementation limit note, including the possibility of OverflowError, and if so, to both len() and __len__() entries. I am not sure of the current doc policy.
The second sentence of the len entry is out of date. " The argument may be a sequence (string, tuple or list) or a mapping (dictionary)." Sets and any collections with a size (__len__ method) can also be arguments. I am not sure how to revise that either.