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#qu…
You won't find it on API:Lists because it's provided by an extension:
<https://www.mediawiki.org/wiki/Extension:PageAssessments>.
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Bartosz Dziewoński