Transferring Pokemon from legacy sources is beyond complicated. In some ways, it's a wonder (trade) you can even do it at all! But thanks to the efforts of Reddit users dominodave and YamiArtio, the path is a little more clear. Well, depending on your perspective.
As we know, Pokemon Home is coming this month and it'll arrive with a few caveats, chiefly: you need to have the premium service to transfer creatures off of the Pokemon Bank (the prior service on 3DS). But there's actually a massive pipeline of transferring opportunities beyond just the Pokemon Bank to Sword and Shield.
Outside of Pokemon Go, Shiny Celebi will become unobtainable when Pokemon Bank closes - until that happens, here's how to get one.
Shiny Celebi was available via movie promotion event. Dexoys is the only one you gotta worry about shiny because it can on be shiny in original gen 3 carts. That being said, they arent closing bank down. This was already stated multiple times.
Shiny Deoxys is currently exclusive to Generation 3, which means it'll become unobtainable once Pokemon Bank closes.
Pokemon Bank is presumably closing soon, and several Shiny Pokemon will become completely unobtainable afterward.