I know there was an event in Japan in 2013 for shiny genesect that would come in a cherish ball. I don't know if it can be transferred to other sources. Also there is an event to get a shiny Genesect in Pokémon go, but is it in a cherish ball when transferred? Overall is it possible to get shiny Genesect in Pokémon Sword and Shield in a cherish ball, or is it hacked?
They're completely legal to use in battle, you just can't get them easily. Game Freak and The Pokémon Company are planning to host various legendary Pokémon events starting in November that will give players access to species like Genesect, Volcanion, and Marshadow.
You can, just requires lots of trading and someone willing to give theirs up. But as listed above there will be an event to get legendaries that you can't find in the game and a Shiny Genesect in a cherish ball would be legal. (The event Pokemon won't be shiny however)
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