FlaggedRevs is installed on the hindi wikipedia as can be seen from [[ https://hi.wikipedia.org/wiki/%E0%A4%B5%E0%A4%BF%E0%A4%B6%E0%A5%87%E0%A4%B7:%E0%A4%B8%E0%A4%82%E0%A4%B8%E0%A5%8D%E0%A4%95%E0%A4%B0%E0%A4%A3 | Special:Version ]]. However, [[ https://hi.wikipedia.org/wiki/Special:PendingChanges | Special:PendingChanges ]] gives a `No such Special Page` error. Also, the entire section of the FlaggedRevs special pages is missing from [[ https://hi.wikipedia.org/wiki/Special:SpecialPages| Special:SpecialPages]]. [[ https://hi.wikipedia.org/w/api.php?action=query&list=oldreviewedpages&ornamespace=0 | An API query ]] shows that there are still pages in the mainspace with unreviewed pending changes. I was able to review one of those listed in the api query result, but still get a `No such Special Page` error.
If I understood [[https://noc.wikimedia.org/conf/flaggedrevs.php.txt | the flaggedrevs config ]] correctly, it seems that its no longer enabled for the mainspace. It used to be enabled for the mainspace AFAIK. Not sure if this is linked to the problem.