Steel was introduced in Pokémon Gold and Silver alongside the Dark type.
The second-generation games expanded the original type chart to add Dark and Steel. Steel-type Pokémon were associated with metal, machinery, and defensive strength, while Dark-type Pokémon represented sinister or underhanded attacks in the Japanese naming context.
The additions changed battle strategy because types determine a move's effectiveness against a Pokémon. Steel gained several resistances, making Steel Pokémon durable, while Dark was designed partly to counter Psychic-type Pokémon, which had been especially influential in the first generation.
New Pokémon such as Skarmory, Scizor, Steelix, and Forretress showcased the Steel type. The games also introduced moves and existing Pokémon interactions that required players to rethink established matchups from Red, Green, Blue, and Yellow.
The Fairy type arrived much later, in Pokémon X and Y, so it is not one of the types introduced by Gold and Silver. Dragon and Ice were already present in the first generation. Gold and Silver therefore added exactly two new elemental categories to the established type system.