On September 10th, 2019, the official Pokemon GO Twitter account released a silhouette of a Pokemon that "is very intelligent and calm and can photosynthesize by bathing its tail in sunlight."
The silhouetted Pokemon is Snivy, the first Pokemon in the Unova generation, and this post is one of the first acknowledgments of the Gen 5 starters arriving in Pokemon GO.
Snivy evolves into Serperior, a mono Grass-type Pokemon that will be quite useful for players who are interested in PvP. At the current rate of Community Days we have now, it will be featured during March 2020 Community Day.
The only other references we've had to the Gen 5 starters so far were a quick Ultra Bonus promotional video posted by the Japanese Twitter account, and the original Pokemon GO Fest 2019 Chicago promotional poster.