MediaWiki logs to the bucket api-feature-usage which ends up in logstash. That represent maybe 1/6 of messages logged and I am not sure whether we need them to be stored in logstash.
There is a special page to query them: https://commons.wikimedia.org/wiki/Special:ApiFeatureUsage
InitialiseSettings.php has it had debug and I have tracked it down to August 2014: https://gerrit.wikimedia.org/r/#/c/154096/
Not sure whether it is still needed?
Similar request for AbuseFilter/StashEdit 146697