During an interview, one of the creators of Pokemon Sword & Shield shared a number of things about the games. One of these subjects was the Limited Connectivity Pokemon Sword & Shield will have with previous Pokemon Games. Due to the amount of work it took to animate the Pokemon (both new and old) that are to be included in The Galar Region, Pokemon not found within the Galar Pokedex cannot be transferred to Pokemon Sword & Shield from previous Mainstream Pokemon Games.
If a Pokemon is found within the Galar Pokedex, it can be transferred from old Pokemon Games to Pokemon Sword & Shield. If a Pokemon isn't found within the Galar Pokedex, then it cannot be transferred to Pokemon Sword & Shield from an old Pokemon Game. This means that hundreds upon hundred of Pokemon will not be able to be used during your journey throughout The Galar Region.