Key Stone appears in 4 version groups since Generation VI, a Gameplay item in the Key Items pocket
PokéAPI item 814 of 2,223, Gameplay category, Key Items pocket
Effect, in PokéAPI's words
Key Stone: “Allows the player’s Pokémon to Mega Evolve.”
Games
Key Stone is listed in the text of 4 version groups, first in Generation VI
Game index by generation
IIIIIIIVVVIVIIVIIIIX
Named in
Sun and MoonUltra Sun and Ultra MoonLet's Go Pikachu and Let's Go EeveeSword and Shield