https://www.gamespot.com/articles/e3-2018-you-can-catch-starters-in-pokemon-lets-go-/1100-6459774/

Nintendo's E3 2018 showing may have been primarily focused on Super Smash Bros. Ultimate, but the company also shared some new details about Switch's upcoming Pokemon RPGs, Let's Go Pikachu and Let's Go Eevee. Among other things, we've learned that players will indeed be able to capture the Kanto starter Pokemon--Bulbasaur, Charmander, and Squirtle--in the wild.

During a hands-on session with Let's Go Pikachu and Eevee, we spotted and captured both Bulbasaur and Pikachu in the wild. The demo was set entirely within the Viridian Forest, so it isn't surprising that we came across Pikachu, as the forest was one of two areas it could be found in the original Red and Blue versions. However, starter Pokemon like Bulbasaur aren't traditionally available in the wild in other Pokemon RPGs.