SwSh
Keldeo is available as part of a Crown Tundra mission.
BDSP
Mew is available if the Switch has LGPE save data.
Jirachi is available if the Switch has SwSh save data.
Arceus is available if the Switch has Legends: Arceus save data with all Missions completed.
Legends: Arceus
Phione is available through a sidequest in normal gameplay.
Manaphy is available as part of the same sidequest as Phione.
Arceus is available in a Mission after completing the Hisui Pokedex.
Shaymin is available if the Switch has SwSh save data.
Darkrai is available if the Switch has BDSP save data.
SV
Meloetta is available in the Indigo Disk.
Pecharunt is available in the Mochi Mayhem epilogue.
Legends: Z-A
Darkrai is available in Mega Dimension.
Keldeo is available in Mega Dimension.
Meloetta is available in Mega Dimension.
Genesect is available in Mega Dimension.
Diancie is available via a Mystery Gift with an unspecified end date.
Hoopa is available in Mega Dimension.
Volcanion is available in Mega Dimension.
Magearna is available in Mega Dimension.
Marshadow is available in Mega Dimension.
Zeraora is available in Mega Dimension.
Meltan is available in Mega Dimension.
Melmetal is available in Mega Dimension.
Mythical Pokemon in the core series (Bulbapedia)
You can get more with Bank, HOME, ORAS, a Gen 7 game, Virtual Console Crystal, and the DNS Exploit in Gens 4 and 5, which can then be transferred to the Switch.
