User blog comment:Brandon Rhea/The Success and Future of Modernization/@comment-25505604-20180127031251/@comment-27345308-20180130100534

There is both a configuration option to allow HTTPS on a certain wiki and a user preference to allow users to enable HTTPS for themselves, as well as a user permission that hides the preference if not present. So if a user is on a wiki with HTTPS enabled AND has enabled HTTPS in their preferences (AND has the permission to do so) they will view Wikia with HTTPS. You can see the actual extension code here.