Swearing would be such an overstep, harassing is subject to opinion, and neither of those are really a dominantly main issue.
If you really wanted to see the toxicity rate drop like an anvil, you'd include the pre-game lobby and the post-game lobby in the chatlogs, which Riot does not currently do, meaning it's really easy to just wait until the end of the game to tell that Ashe how much of a grilled dick sandwich she ate the entire game as she fed the fuck out of Jinx, or that Teemo top how much of a bastard he is for, you know, just picking Teemo, because that's totally justified.
The other thing is there is no accountability for if the entire team is toxic. So if you have all four people attacking you for feeding top-lane, doesn't matter if the enemy team agrees that your team had shafted you by calling no MIA's and your jungler never bothering to gank a pushed-to-turret Darius which proceeded to tower dive you - trying to report four people lessens the value of your reports, so you'd basically just have to choose one unfortunate bastard to carry all the blame if you wanted anything to happen. Of course, there's a line between "toxic" and your team legitimately just going, "Ashe, you need to stop trying to duel the enemy Jinx; she's way more fed and she will win every fight. Just group with us." And then the Ashe says, "Stfu". And then you reply with, "... Seriously?" And then the Ashe says, "Look at Akali, she's super fed also." To which you reply, "Jinx has 10 kills to Akali's 6, not to mention 4 of those were off of botlane and the other two were because you didn't even bother to help the jungler and I take drag." And then for some reason you get reported for negative attitude after the game because trying to explain to an Ashe how she's being an ass is a bannable offense. Hell, maybe that Ashe was right in saying that she was crazy camped and our jungler never bothered to gank for her once. Or maybe in actuality the jungler ganked three times and gave up because the first time she didn't follow up and the other two she completely missed her ult. I'm just speculating here.
... Point is, this shit is super easy to write out on paper. Practicing it all can get way more problematic.