Summary:
On Thursday July 11th we will be enabling a new centralized login
service to make the login experience for users more streamlined.

More detail:
To address the shortcomings of the current Single User Login (SUL)
system, the Platform team is making significant changes to the way that
CentralAuth logs users into other wikis on login:

* Users will be logged into a new, centralized domain when they login to
  any public WMF wiki with a global account. This domain is
  login.wikimedia.org
* Global accounts will no longer see the "Login Success" page after
  login, instead they will be redirected back the article where they
  came from. On the article page, we will transparently attempt to login
  the user to all of the sister projects, instead of relying on the
  images on the login success page.
* All of the public WMF wikis will use the central wiki to check
  anonymous user's logged-in status, and transparently log the user in
  if they are centrally logged in.

(Thanks to Chris Steipp for the above language.)

You can see the full rollout plan for this here:
https://www.mediawiki.org/wiki/Auth_systems/SUL2#Rollout_Plan