This appears to be a bug. I filed https://phabricator.wikimedia.org/T186648.
The API should still be available, it is listed here: https://en.wikipedia.org/w/api.php?action=help&recursivesubmodules=1#que...
You won't find it on API:Lists because it's provided by an extension: https://www.mediawiki.org/wiki/Extension:PageAssessments.