[[OSM:Notifier.signup confirm html.user wiki page/en]]

Not sure what you mean by that Activerecord.models.acl reference, I don’t see anything interesting there.

However, you might be right that the Osm:Notifier.signup_confirm_html.user_wiki_page message is no longer used in production OSM, the current welcome message does not include it, and I don’t think it could appear somewhere else, but I can’t be sure. If the message is indeed no longer used, it could/should probably be removed from the OSM source code altogether.