A long time ago, the Wikibase data model apparently knew a fourth rank: “truth”. This was deprecated and removed sometime between 2014 and 2015, but still survives in the mw.wikibase.entity Lua module as one of the available claimRanks. We should probably get rid of it there.
Unfortunately, though, it’s used in several modules (many of them copy+pasted, it seems):
lucaswerkmeister-wmde@mwmaint1002:~$ mwgrep --module RANK_TRUTH brwikisource Module:Central-fr-ws brwikisource Module:Centralizer dewikisource Module:Central-s-de elwiki Module:Carte interactive eswikisource Module:Central-s-es frwiki Module:Carte interactive frwiki Module:Central frwiki Module:Central-s-fr frwiki Module:Centralizer frwiki Module:ControlArgs frwiki Module:Tests frwikisource Module:Tests frwikisource Module:Tpt frwikiversity Module:Centralizer huwiki Module:Central-w-hu mediawikiwiki Module:Central mediawikiwiki Module:Central-mw-en test2wiki Module:ControlArgs viwiki Module:Centralizer (total: 19, shown: 19)
Some of those might be okay if mw.wikibase.entity.claimRanks.RANK_TRUTH turns into nil, but I haven’t checked all of them.