Fairy does ×½ to Steel

Generations VI to IX, single types. The other way round, Steel against Fairy, is ×2.

64 Fairy and 65 Steel species among the 1025 this site covers.

Attacker, Psychic and Fairy Hatterene sprite

damage×½Generations VI to IX

Defender, Rock and Steel Stakataka sprite

Fairy and Steel did not exist in Generation I and Generations II to V, so Fairy against Steel has no value there.

Strongest Fairy attackers

Hatterene leads Fairy at Sp. Atk 136

Sturdiest Steel defenders

Dual types that change the answer