Nearly all of its world map was a free-roaming open area, but the sections were still cordoned off from one another by way of a central hub town. Pokémon Legends: Arceus came even closer.
Those games and their DLC packs introduced a few dedicated free-range areas where you could roam around mostly at your leisure, but towns and caves and other areas were still strictly linear. The Pokémon franchise has been nudging closer and closer to a fully open world since Sword and Shield appeared on the Switch in 2019.
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