Last modified: 2014-06-27 07:28:45 UTC
AFAIK disabled for performance reasons. "Shows a list of people who contributed to the page in question using the real name specified in the user's preferences. Currently disabled on Wikimedia wikis, but is enabled by default on vanilla MediaWiki." https://translatewiki.net/w/i.php?title=Translating:MediaWiki&action=credits
Indeed disabled via: $wgActions['credits'] = false;