Which Pokémon games introduced the Dark and Steel types?

Pokémon Gold and Silver introduced the Dark and Steel types.

Released for the Game Boy Color in Japan in 1999, Gold and Silver were the second-generation main-series Pokémon games. They added 100 new Pokémon, bringing the total from the original 151 to 251. Dark and Steel joined the original type system as new categories.

The new types helped rebalance battles. Steel Pokémon generally gained strong defensive advantages, while Dark Pokémon were designed partly to counter Psychic-type Pokémon, which had been especially powerful in the first generation. The games also introduced new moves, abilities in later games, breeding and a day-and-night clock.

Pokémon Crystal followed Gold and Silver as an enhanced version. Ruby and Sapphire later added the third generation, but they did not introduce Dark or Steel. The types first appeared in the second-generation pair, making Gold and Silver the canonical answer.