Contest Costume appears in 3 version groups since Generation VI, an Unused item in the Key Items pocket
PokéAPI item 934 of 2,223, Unused category, Key Items pocket
Effect, in PokéAPI's words
Contest Costume: “XXX new effect for contest-costume--dress”
Games
Contest Costume is listed in the text of 3 version groups, first in Generation VI
Game index by generation
IIIIIIIVVVIVIIVIIIIX
Named in
Ultra Sun and Ultra MoonLet's Go Pikachu and Let's Go EeveeSword and Shield