Blizzard has published a new blog post reminding players to migrate their account security to the newer Battle.net mobile app, as the standalone Authenticator app will soon be removed from accounts and any accounts still using it will have their passwords reset. The authenticator itself will be removed from the app store on January 1st, but still be available for use until the 5th, at which time the migration needs to be completed to avoid any disruption of service.
The authenticator was merged into the Battle.net mobile app last April, which also allows chatting and other social functions. Other than that switch to a new mobile application, authenticating should continue to work much in the same way as before though — authorizing log-in, unlocking accounts, resetting passwords, and so on. As an additional bonus, players who haven't already added an authenticator to their accounts will receive the Core Hound Pup battle pet and four extra primary bag slots for all their retail WoW characters once set up.