Pokémon Scarlet and Violet will feature regional variants, but some may not appear in the Pokédex according to one leak. Regional variants were introduced to Pokémon in Generation 7 and continue to be featured in the newest iteration, Pokémon Scarlet and Violet. When first introduced in Pokémon Sun and Moon the Alolan variants changed Pokémon that have been around since Generation 1 into completely different types such as the Alolan variant of Ninetales in Sun and Moon being changed from a Fire-type to an Ice-type. These variations would be mentioned in the Alolan Pokédex, occasionally explaining the regional variations such as changing the entry for Raichu from having electricity to psychic energy.
The regional variations continued into the following generation with Pokémon Sword and Shield having regional forms such as Rapidash becoming a Psychic/Fairy-type and Rattata becoming a Dark-type. Some Pokémon have received variations across more than one generation, like Meowth becoming a Dark-type in Alola and a Steel-type in Galar. These regional variations have continued into the ninth generation with the confirmation of Pokémon SV's Paldean Wooper changing it from a Water- to Poison-type.
It was recently claimed by Centro LEAKS on Twitter that there is a possibility some regional variants from previous games within Pokémon Scarlet and Violet will not be in the Pokédex. The tweet states that players will be able to obtain a Galarian Meowth by an in-game trade where they will need to relinquish a Pawmi, but the variant will not be mentioned within the game's Pokédex. In previous installments, any regional variants had pages within the Pokédex which were changed to reflect the new appearance and type changes for each one.
Pokémon SV's Regional Variants Are Still A Mystery
It is currently not confirmed by Game Freak if this leak is true or if other regional variants will have their own pages within the Paldean Pokédex, or even if all the variants from previous games will be available in the new generation. Quite a few Pokémon that have gotten a regional variant are from Gen 1, including the Legendary Birds, and there has so far been no confirmation for which Pokémon will be receiving variants in Scarlet and Violet, as at the moment, only a fraction of the total Pokémon have special forms.
Pokémon has expanded from 151 monsters to over 900, and the introduction of variations has helped to build the world into one more reflective of real life, with areas having their own forms of wildlife that are similar but different enough to distinguish them as unique. If players can obtain other variants than just Paldean within Pokémon Scarlet and Violet by trade, it would not be the first time regional variants were not in another game's Pokédex, as Hisuian variants weren't in Pokémon BDSP, even with Legends: Arceus canonically happening prior.
Source: Centro LEAKS/Twitter