Pokemon only obtainable through transferring
Diancie
Hoopa
Magearna (While its Original Color form is obtainable through Pokemon Home as a gift Pokemon, the requirements of obtaining it is to complete the National Dex, which requires the use of Pokemon games outside of the Switch titles)
Melmetal (Similarly to Magearna, Melmetal is a gift Pokemon in Pokemon Home as well. However, it still requires the use of transferring any Pokemon in from Pokemon Go)
Pokemon completely unobtainable in any Switch titles
Note that all of these Pokémon have been confirmed for Legends Z-A
The Patrat line
The Pansage line
The Pansear line
The Panpour line
Furfrou