Shedinja is currently available as the weekly Field Research Breakthrough in Pokémon GO. This Pokémon has only ever been available through research and, despite the Candy needed to power it up being classified as Nincada Candy, Nincada cannot evolve into Shedinja in the game. What's the deal with this strange Bug/Ghost-type Pokémon? To find out, we look to the main series games.
The Nincada evolutionary line in Generation Three, which brought the Hoenn region into the world of Pokémon. These Bug-type Pokémon are based on cicadas, a species of insects that moult, shedding their skin so that their adult form can emerge and fly away. In the world of Pokémon, Nincada's evolution creates two separate species: the emerged adult, which is Ninjask, and the abandoned, dead husk of its exoskeleton, which is Shedinja. Hence, its dual typing as a Bug/Ghost-type Pokémon. In the main series games, players would leave an open space in their party when evolving Nincada and, when the Nincada had successfully evolved into Ninjask, the empty space would be filled by Shedinja.
This is not how it works in Pokémon GO. In Niantic's mobile take on this whimsical world, Nincada evolves into Ninjask, and that's the end of the story. Shedinja can only be captured when featured in research. However, there is an interesting way that Niantic could implement this mythology-driven evolution.
Photobombs are currently active in Pokémon GO. Outside of events, Smeargle will, at random, appear in GO Snapshots. After the picture is taken, a Smeargle pops up on the map and can be encountered and caught. Other species are featured during events, such as this current Fashion Week's Croagunk photobombs. This mechanic allows a player to trigger a spawn… so why couldn't this be done through evolution? A mechanic where evolving Nincada into Ninjask initiates a Shedinja to spawn on the map perfectly creates the mythology that the original games used for these Pokémon. Introducing this mechanic would also be a terrific way to release Shedinja's Shiny.
It's uncertain if Niantic will do anything like this, but it's safe to say that there has to be a plan for Shedinja in the future. With no Shiny release during this current rotation of the Research Breakthrough, it's clear that they're holding it back for something. What that something is, though, can only be guessed.