Basement Key appears in 3 version groups since Generation VI, an Unused item in the Key Items pocket
PokéAPI item 931 of 2,223, Unused category, Key Items pocket
Effect, in PokéAPI's words
Basement Key: “XXX new effect for basement-key--new-mauville”
Games
Basement Key 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