It is a common convention for all output iterators (that are not also input iterators) to have 'void' for all their typedefs other than the iterator category. If you think there is an issue here, it should be filed with the Library Working Group chair, as it is distinctly more than editorial - but I think the standard is exactly as intended here.

The only value operation to perform when dereferencing an iterator with category output_iterator is to assign to it, so a void value type discourages people from trying to do:

iterator_traits<ostream_iterator<int>>::value_type = *ostr_iter;

Of course with auto you don't need to know the value type, but that doesn't mean that a non-void value type would be useful.

As Alisdair says, this is not an editorial issue, so does not belong here.