The August edition of Japanese magazine CoroCoro has leaked and confirms that Mega Evolution is available in the upcoming games Let's Go Pikachu and Let's Go Eevee. The original Kanto starters are shown with their Mega Evos:
Mega Venusaur, Mega Charizard X&Y, and Mega Blastoise in Pokémon Let's Go
There is no confirmation of hold items, though they would not be strictly necessary.