Ubisoft removed the shotgun headshot multiplier in Rainbow Six Siege Operation: Red Crow, meaning that that, regardless of aim, every bullet in the spread deals the same as if you shot your enemy in the body. You'll most often only be able to see your opponent's head while the rest of the body is behind cover. Because of this, shotguns are unable to compete with other weapon types in Rainbow Six Siege — shotguns need to the headshot multiplier back.
Take a look at this gif below for an example.
Any other weapon would have one-shot the Fuze with no chance of him going into the down-but-not-out state. Due to the multiplier being taken away, he was only able to do about 60 percent damage.
Ubisoft removed the shotgun headshot multiplier in Siege because there was an absurd amount of people getting killed by a single shotgun pellet to the face. It even influenced the meta at one point to run five GSG Recruits and rush. Granted, that was seen more in the casual game mode as a way to troll. However, the troll-meta had significant success in Competitive.
That being said, there were other options to scale back the shotgun massacres. A simple reduction to the headshot multiplier would have made more sense. It would have reduced the amount of shotgun insta-kills and forced the players using the weapon to aim with more precision.
Instead, the viability of shotguns Rainbow Six Siege was completely flipped and now a majority of players just opt to avoid the shotgun due to the over-nerf.
Unfortunately, we may not see a change to the shotgun for some time as the Siege developers have larger problems on their plate: hit detection and server stability. So far, there's been minor updates to hit detection included in Operation: Velvet Shell and the latest Mid-Season Reinforcements patch, though it's still not ideal.
Similarly, Ubisoft has begun work on bettering the stability of the servers through Technical Test Servers. The test environment should provide Ubisoft with ample data on coming adjustments including the new One-Step Matchmaking system.
Hopefully we will see a change to the shotgun headshot multiplier in the Hong Kong operation for Rainbow Six Siege which is slated for May of this year.