https://www.polygon.com/pokemon-go/2018/6/22/17493364/an-alolan-favorite-returns-to-the-wild-in-pokemon-go

Earlier this month, Pokémon Go players mourned the loss of the biggest and best wild, Alolan-form Pokémon: the Alolan Exeggutor. In late May, an Alola-themed event rolled out for the announcement of Pokémon: Let’s Go, Eevee! and Pokémon: Let’s Go, Pikachu!, featuring Pokémon Sun and Moon’s regional variants of Kanto Pokémon. Alolan Exeggutor charmed fans with its sheer size. It often dwarfed Pokémon that should be larger than it and could not be fully captured in the AR view.

The event wrapped up in early June, and the Alolan forms disappeared from the wild, Alolan Exeggutor among them.