There are ways around it and only the dumbest of the dumb get caught.
Machine learning is not just string processing and checking if you said a few bad words or known-in-advance sentences... It's taking a whole lot of cases known to be problematic (through reports), and automatically finding the similarities between them to learn what is the problem in those cases. Then you can look at a new game and decide if there is the same problem in it. There isn't a fixed set of stuff the system can detect, it can adapt dynamically if players start to report different things.
You're never punished for one single game, though. Seeing the same behavior exhibited by different persons a few times in the same month proves nothing. And people still have to do something bad BEFORE being punished, you can't just guess they will be bad in the future and remove them beforehand. So even if the system was perfect, or even if there was a Tribunal, there would still be games with flamers and ragers.
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If they actively follow the jungler, that's dumb easy to detect. If they "steal" on the other side of the map, a bit harder, 'cause laners can have good reasons to do the same, but if they are doing it on purpose all game long, that's not too hard either.
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Easily detectable, just not serious enough to be punishable for just that.
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Same thing, you even admit it yourself, it's damn easy to detect. If your "friend" has not been punished yet, it's not a problem of detection, it's just that it's not considered serious enough.
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Alternate spelling is not a problem, especially if people do it often, as you claim it. As I told in my first post, machine learning is pretty powerful, it just needs enough examples to understand what's the problem. But something as straightforward as alternate spelling is, once again, damn easy. You can detect that "st00p1d" is an insult just the same way that you detect "stupid" is an insult to begin with.
Sure, there were toxic people back then, but far fewer than now and they were dealt with far more abruptly
Riot said that's not true. I never played when the Tribunal was active, but as far as I am concerned, negativity and toxicity have decreased since I started playing. There could be plenty of reasons for that outside of just toxicity decreasing in general, that would make my experience not representative, but still... I tend to believe Riot, 'cause from what I can see, I have no reason not to.