https://www.gamespot.com/articles/pokemon-lets-go-pikachu-eevee-bring-back-mega-evol/1100-6461017/

Alolan forms won't be the only Pokemon variants available in Pokemon: Let's Go, Pikachu and Let's Go, Eevee. The latest issue of CoroCoro magazine (via Pokemon fansite Serebii) has revealed the upcoming Nintendo Switch games will also feature Mega Evolutions, the powerful transformations introduced in Pokemon X and Y. [Update: The Pokemon Company has shared a new English trailer for the Let's Go games, showcasing Mega Evolutions in action.]

Details are scant at the moment, but it appears Mega Evolutions will function much the same as they do in X and Y and their follow-ups, Pokemon Sun and Moon. In those titles, players need to equip a Mega Stone to a corresponding Pokemon, which will allow it to Mega Evolve once per battle. Along with taking on a new appearance, Mega Evolving often changes a Pokemon's type and gives it a significant power boost.