All Starter Pokémon will return in Pokémon Scarlet and Violet: The Hidden Treasure of Area Zero DLC
At the closing ceremony of the 2023 Pokémon World Championships, it was announced that all Starter Pokémon will be returning to Scarlet and Violet in Part II of The Hidden Treasure of Area Zero DLC.
This means Starter Pokémon from all previous regions will be obtainable in Scarlet and Violet when The Hidden Treasure of Area Zero: The Indigo Disk is released in Winter 2023. It looks like all of them will be roaming around the various areas within the underwater eco-dome of Blueberry Academy, but it is currently unclear how you will encounter them.