In https://gerrit.wikimedia.org/r/c/operations/mediawiki-config/+/220970 wgCentralAuthAutoCreateWikis was initially populated with 'loginwiki', 'metawiki', and 'mediawikiwiki'.
- 'loginwiki' was added for central account creation login and checkuser requests on it (T85703).
- 'metawiki' and 'mediawikiwiki' was added for oauth reasons (T94885 T74469). 'mediawikiwiki' was the historical wgMWOAuthCentralWiki, and has been replaced by 'metawiki' (T108648 https://gerrit.wikimedia.org/r/#/c/233609/).
So I think 'mediawikiwiki' is no longer needed and can be removed from auto account creation wiki list.