After defeating a Rocket grunt, you get a chance to catch their left-behind Shadow Pokémon. Some of these (e.g. Poliwag, Dratini) can be Shiny if encountered in the wild, after a raid, by field research, or hatching from an egg. Pikachu with special hats can also be Shiny.
Can Shadow Pokémon be Shiny? If yes, do they have the usual 1:450 Shiny rate?