Even with all of the legendary Pokémon that have been added to Sword and Shield in The Crown Tundra expansion, there are still certain encounters that aren’t actually available in-game.
This only applies to legendary and mythical Pokémon that aren’t actively available in main series games and were distributed through special events or giveaways.
Two of the more prominent Pokémon that fit that description are Volcanion and Diancie, two generation six Pokémon that have only been available as a part of events. This means you won’t be able to encounter them anywhere in Sword or Shield, not even in the Max Lair where Dynamax Adventures take place.
The only way to legally obtain both Volcanion and Diancie right now is through using Pokémon HOME.
If you previously obtained either Pokémon through events in Sun and Moon, you can transfer them through Pokémon Bank into HOME and then send them over to Sword and Shield. They’re completely legal to use in battle, you just can’t get them easily.
Game Freak and The Pokémon Company are planning to host various legendary Pokémon events starting in November that will give players access to species like Genesect, Volcanion, and Marshadow. That specific event will run in Japan on Nov. 20, but it’s not yet known if the codes will work globally.
Future events like that will likely be held for other Pokémon like Diancie, but no details have been announced yet.