Well, it was released in the Japanese eShop however due to a high volume of users downloading Nintendo Network IDs and downloading the app the eShop had crashed.
After the crash, the PokeBank + PokeTransporter was nowhere to be found. They are still not there.
Only very few amount of people have got the app on Japanese 3DS systems, however no one else has yet to get the PokeBank app. So, it is not out yet and it is unconfirmed when it will be released again.
Hope I helped. :)