Steel was added in Pokémon Gold and Silver alongside the Dark type.
These two types debuted in Generation II, which was designed partly to rebalance the battle system introduced by the first-generation games. Steel-type Pokémon are generally associated with metal, armor, and high defensive strength, while Dark-type Pokémon were designed around dark or underhanded techniques.
The addition of Steel changed the type chart by giving several older Pokémon new strategic relationships. Steel is super effective against Ice and Rock, while it resists many types. It is also weak to Fighting, Fire, and Ground attacks under the modern type chart.
The Fairy type did not appear until Generation VI, beginning with Pokémon X and Y in 2013. Dragon and Rock were already present in Generation I, so they are not answers to this Generation II question. Type interactions have been revised slightly across the series, but Steel’s debut remains a defining feature of Gold and Silver.