There are so many Pokémon to be found in Brilliant Diamond and Brilliant Pearl that players will want to prioritize their wish lists from the very beginning of the game. For many, Shiny Pokémon will be one of the most important things they will try to find in the game.
Others will want to start their mythical collection by grabbing Mew and Jirachi as fast as possible. You can get them by saving Let's Go Pikachu / Eevee and Sword and Shield on your Switch. Before doing so, they will want to know if those Pokémon can be Shiny. Sadly, both Mew and Jirachi are Shiny locked, so there's no point in manually saving and then reloading the encounter the Pokémon brings you. No matter what you do, they won't be Shiny variants.
Manaphy is no longer difficult to obtain. You will have to visit the TV building in Jubilife City and go to the third floor and talk to an NPC there to get access to the mystery gifts and you will receive a Manaphy egg. Manaphy is also Shiny blocked, so there's no reason to try joking in your favor.
Now, keep in mind that it's still in its infancy and it may turn out that Shiny versions are available, but none have appeared at the moment and they really seem to be Shiny blocked.