Steel was introduced as a new Pokémon type in Pokémon Gold and Silver and is immune to Poison-type moves.
Generation II added both Steel and Dark, expanding the battle system beyond the types available in the original games. Steel was designed around metallic strength and resistance, and Steel-type Pokémon generally became known for high defensive capabilities.
In the Generation II battle system, Poison-type moves do not affect Steel-type Pokémon. Steel also resists several other types, although its exact type matchups have been adjusted across later generations. These changes helped counterbalance the power of some earlier strategies, especially those involving Psychic-type Pokémon.
Dark is the other new type from Gold and Silver, but it is not the answer to this question. Dark was introduced partly to provide a strong answer to Psychic-type Pokémon. Because both types debuted together, players sometimes remember the pair but mix up which one has immunity to Poison moves.