Is trapezius slimming dangerous and why isn’t it more common?

I’ve been interested in trapezius slimming via injections for a while now as my traps are very overdeveloped due to bad posture which makes my neck and shoulders seem short and bulky. However I haven’t been able to find any doctors near me who have experience with this. However I know that it is more common in places like Australia. What are the risks with this procedure and what is used to accomplish the slimming? Is it Botox or some other injectable? Why is it uncommon?