Whereas en.wikipedia.org gives me an answer:
https://en.wikipedia.org/w/api.php?action=query&titles=MediaWiki:Cite_error_ref_numeric_key&prop=revisions&format=xml&rvprop=content|ids|user
www.mediawiki.org does not:
https://www.mediawiki.org/w/api.php?action=query&titles=MediaWiki:Cite_error_ref_numeric_key&prop=revisions&format=xml&rvprop=content|ids|user
I get:
<api batchcomplete=""><query><normalized><n from="MediaWiki:Cite_error_ref_numeric_key" to="MediaWiki:Cite error ref numeric key"/></normalized><pages><page _idx="-1" ns="8" title="MediaWiki:Cite error ref numeric key" missing=""/></pages></query></api>