Regular Magearna does not have a shiny. However, its alternate form, Original Color Magearna, does. Unfortunately, it is shiny-locked and therefore unobtainable.
Source<---- This shows that Magearna is shiny locked.
Source<---- This shows that Magearna has an alternate form.
Source<---- This is a video by a PokeTuber that I watch where he goes through the full list of shinies. At 26:15 he gets to Magearna and you can see the Original Color form and its shiny, proving that it has a shiny.