On Wikimedia sites, the post-account-creation page (with 'welcomecreation-msg', etc.) is never shown.
This is quite problematic, because many wikis override that message to provide all kinds of welcoming information. For example: https://pl.wikipedia.org/wiki/MediaWiki:Welcomecreation-msg The only welcome message the user gets is Echo's 'notification-new-user'. This can't possibly be a good thing with the editor decline and all that.
When this message was being changed a few years ago, there was a metric ton of announcements (http://www.gossamer-threads.com/lists/wiki/wikitech/317345). Now it just disappeared.
I have no idea what is causing this. Possibly CentralAuth or GuidedTours? Primary suspects are extensions using the UserLoginComplete and BeforeWelcomeCreation hooks.