The authentication doc page now lists URL's like:
While these seem to work, the correct domain is
stackexchange.com, as previously listed in the doc page (and still works).
The API is network-wide, not limited to Stack Overflow.
The doc page seems to have been silently reverted to
stackoverflow.com around April Fool's day of last year. (March 14, 2018 was still good, but the page had been regressed by April 14, 2018.)