If you were hoping Game Freak would add the Pokemon missing from Sword and Shield in a future update, consider those hopes squashed. In an interview with InsideGamer, Game Freak's Junichi Masuda said their are no plans to add remaining Pokemon to the game. On top of that, Masuda states that this approach is one they'll take for future Pokemon games as well. Check out his full statement below.
“We currently have no plans to make the Pokemon that are missing from the Galar Pokedex available in-game. This is an approach that we want to continue with in future Pokemon games. Up until now it hasn’t been possible to encounter every Pokemon in every game, so people had to transfer them from old games to the new game, by using Pokemon Bank for example.”
“The Pokemon Home app is currently in development, in which players can collect their various Pokemon, and only Pokemon in the Galar Pokedex can be transferred from there to Sword and Shield. But the way of playing is actually not very different from before with Pokemon Bank: up until now you have always only been able to meet Pokemon from a certain region.”
“We encourage people to use Pokemon Home to collect their Pokemon from old games. From there they might be able to take them to other games in the future. So take good care of your old Pokemon, because perhaps you can travel with them again in the future.”