Super rare.
The couple times I've seen it, the whole lobby quickly turns against the player doing it.
It seems that even your own team turning hostile to you when you cheat in deadlock is a pretty good deterrent.
And once your team stops helping you, and you fall behind on farm, it doesnt matter how good your aimbot is, you die a lot.
Cheating in Deadlock is probably usually more map vision or wallhack type stuff. That's much more beneficial than automatic aim, and much harder to spot.