So if I am not mistaken the code of conduct applies to all players. However it is visible only in the forum which has the result of most active multiplayer players not knowing about it. Seriously do you know how common it is for people to kick because of others lagging, camping, being targeted, being killed or just for the heck of it. If you tell those people about Code of Conduct they won't aknowledge it since they have never even heard of it.
This has likely been suggested before by others. Like with the chat and the forum players should agree on following the code of conduct before having the ability to enter multiplayer. I don't really expect most players to read it or anything ( most players will probably just skip it) but at least they will know of it's existence. the knowledge of the existence of rules should decrease the amount of toxicity in game since most players think rules were made by the society.