"The Pokémon Company has revealed the schedule for the official release of the four password stages in Pokémon Picross in Japan. These stages unlock Pokémon not normally available within the gameplay of the game, but have been documented in our section
Zygarde 10% Forme's password will be given at the Seven Spots in 7/11 stores across Japan from December 15th through to January 31st
Ash-Greninja's password will be given in the Pokénchi episode that airs on December 20th
Mew's password will be given in the Famitsu issue released on December 24th
Complete Forme Zygarde's password will be in next month's issue of CoroCoro on January 15th"

So we're getting a reveal in corocoro or in january, 3 years after XY announcement.