I remember having a shiny starter with swsh.
And they can’t be shiny locked if you force them to be shiny. When I need to retrieve the starters from the professor I did catch them shiny. But when picking one before starting the trial they are normal again. Can’t seem to find a way to spawn them shiny. This also because you can’t leave the room when you get the chance to shoose
Might have to check if they pass the legality check then, especially in Sword/Shield