Question: is zacian and zamazenta shiny locked?
Answer: All three of the starter Pokémon—Grookey, Scorbunny, and Soble—were shiny locked at the start of Sword and Shield, so resetting the game won't change the encounter rates at all. The same can be said of all NPC trades, Wild Area Pokémon that can't be captured, and the Legendary trio of Zacian, Zamazenta, Eternatus.