We probably have a long, long time to wait until Niantic adds Shiny Galarian Slowpoke, Slowbro, and Slowking to Pokémon GO. However, that doesn't mean that we can't take a peek. In face, some of you may actually be able to see Shiny Galarian Slowpoke, Slowbro, and Slowking in your actual app. Let's get into it.
Pictured above, you can see the regular and Shiny comparisons of all three forms. While not yet released in Pokémon GO, they are in the code, which allows dataminers to show us what they'll look like in-game. The Shinies are pictured to the right, with Galarian Slowpoke going a full mustard yellow, Galarian Slowbro switching out its bright punk for a creamsicle orange, and Galarian Slowking's gem changing from green to red, its frill turning blue, and its body becoming more bright and saturated. Overall, while not as stunning as previous Galarian Shinies like Ponyta and the Zigzagoon line, it's still quite nice.
Now, even though they're not released in Pokémon GO, you will be able to see these only if you have caught a Shiny of their Galarian counterpart. For example, if you have Shiny Slowpoke as originally discovered in the Kanto region, go over to your Pokédex. When looking at the Dex, you'll notice that the Shiny button next to Galarian Slowpoke is clickable. Click it, and it'll show an unlocked entry. If you have evolved your Galarian Slowpoke into Slowbro, you'll be able to see Shiny Galarian Slowbro. The same holds true for Shiny Galarian Slowking.
The glitch essentially unlocks any regional Shiny if you have one from any available region. If you've caught Alolan Diglett, you can see Shiny Kanto Diglett. If you've caught Kanto Meowth, you can see Alolan Meowth and Galarian Meowth even though the latter hasn't even been released as a Shiny yet in Pokémon GO.