Contest Costume appears in 4 version groups since Generation VI, a Gameplay item in the Key Items pocket
PokéAPI item 780 of 2,223, Gameplay category, Key Items pocket
Effect, in PokéAPI's words
Contest Costume: “Worn during Pokémon Contests.”
Games
Contest Costume 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