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

Yes trapezius slimming with Botox is a thing. I do not perform this off label procedure myself because it would require using significantly higher doses of botulinum toxin than we typically use for cosmetic treatments. It is not unusual to use higher doses for therapeutic treatments, but for a cosmetic treatment, I don't think it is worth the cost or the risk which include unintended paralysis of surrounding muscles and developing an immunity to the product. 

Hi there, Trapezius slimming would be done with a neurotoxin, like botox as you mentioned. As this is a larger muscle group it would required a very large amount of botox and may require several treatments, making it a very expensive procedure. Also this is not an approved treatment by Health Canada, and would be an "off-label" use of a neurotoxin. The risk is muscle weakness and "animation deformity," meaning impaired use and functioning of the muscle. Given that the trapezius is a very important muscle for the neck, shoulders, upper back and head, I would encourage you to very carefully weigh the risk vs. benefit of the procedure.