Last modified: 2013-04-18 16:39:17 UTC
I'm not sure at the moment whether it was only disabled or it actually doesn't exist, but I checked this on wikidatawiki, enwiki, mediawikiwiki, and foundationwiki, and I get the no such special page message. In either case, it is still linked from Special:Statistics, and so should be removed (conditionally if disabled, entirely if removed, or the active users page replaced if that was the only issue).
Special:ActiveUsers was removed for performance reasons (bug 46719), and the link on Special:Statistics was mistakenly left there (bug 41078). The fix has not yet been merged or deployed. *** This bug has been marked as a duplicate of bug 46719 ***
*** This bug has been marked as a duplicate of bug 41078 ***
*** Bug 47364 has been marked as a duplicate of this bug. ***