Which Pokémon games introduced the first open-world main-series setting?

Pokémon Scarlet and Violet introduced the first open-world setting in the main-series Pokémon games.

Released for Nintendo Switch on November 18, 2022, the games are set in the Paldea region, which is inspired by the Iberian Peninsula. Their world combines urban areas, wilderness, mountains, lakes, and other environments without the conventional loading boundaries between routes and towns.

Players can explore Paldea freely and pursue three major story routes: Victory Road, Path of Legends, and Starfall Street. Gym battles can be tackled in different orders, although the games do not automatically scale every challenge to the player's level. Koraidon or Miraidon serves as a mount for traversal.

Pokémon Legends: Arceus is sometimes named as the answer because it features broad open areas and released earlier in 2022. However, Scarlet and Violet are described as the first main-series Pokémon games with a continuous open world, while Legends: Arceus uses separate large zones.