From the PR:
So that's Sun & Moon, Ultra Sun & Ultra Moon, Shuffle and now GO that have been invaded by this.
No word on which hat, but it's most likely the movie hat
To celebrate Pokémon GOs one-year anniversary, Niantic and The Pokémon Company today revealed that from now through July 13, Trainers will be able to find and catch Pikachu wearing Ashs famous hat from the Pokémon animated series. A new featured store item, the Anniversary Box, which contains Incubators, Max Revives, Ultra Balls and a Raid Pass, will also be available for a limited time through the in-game shop at a discounted price.
So that's Sun & Moon, Ultra Sun & Ultra Moon, Shuffle and now GO that have been invaded by this.
No word on which hat, but it's most likely the movie hat