Hello, Shiny magearna doesn't legitimately exist, unless you are referring to original colour magearna? Original colour Magearna is the gift you receive in pokemon Home for completing the full pokedex, all 905 (and gen 9 dex after that).
It probably doesn't mean much, but you only need Pokemon up to Eternatus (#890) to get Original Color Magearna. Everything introduced in SwSh DLC, Hisui, etc. isn't required.