Considering the purpose of it is to inflict harm upon a person's body and do no less damage than a brass set, then the obvious answer would be that they are illegal and do fall under the category of prohibited weapons.
If you shot and killed someone with a plastic bullet, would that be illegal?
I know this question is old but I have to comment when people don't take the time to read law and give bad information. According to CBSA (don't have a link but I have a print copy) "Plastic knuckles do not fall within the definition of brass knuckles and are therefore not a prohibited weapon" so this means they are not a prohibited weapon. however; I can guarantee if you are caught with them concealed you will be getting a fine for concealed weapon. I was caught with a legit brass set of knuckles (while on weapons prohibition) had to go to court and was fined 500$ for the posession of a prohibited weapon. Moral of the story metal knuckles = prohibited... plastic knuckles= non prohibited but can still land you in hot water as they are still a weapon. So you can buy them but you can't walk around with them.