https://www.polygon.com/2018/10/9/17955658/pokemon-go-sinnoh-region-gen-4-updates

Your Pokedex in Pokémon Go is about to get much bigger, as creatures from the Sinnoh region make their way to Niantic and The Pokémon Company’s mobile game. A new trailer for Pokémon Go teases the arrival of the first fourth-gen Pokémon, including starter trio Chimchar, Turtwig, Piplup and the legendary Giratina.

Sinnoh-region Pokémon made their series debut in 2006’s Pokémon Diamond and Pearl, and they’ve been a long time coming in Pokémon Go. Niantic and The Pokémon Company didn’t provide a specific date for the arrival of Sinnoh-region Pokémon, but their release appears imminent.