I think Riot should
Plaster a list of badwords you can't say to others playing this game.
Ignoring hate speech, there's never really going to be a definitive list of bad words for people to avoid. Even things that one could easily assume to be punishable can be acceptable within certain specific contexts; I.E, you can't go around calling people idiots, but you can say "I'm playing like an idiot right now, sorry guys".
And even with hate speech, it's altogether easier and more cost-effective to give broad, general statements about what behavior isn't allowed than it is to try and specifically list off everything you can and can't say. Especially since, having a definitive list of behaviors you can't do will almost certainly encourage some people to just come up with new ways to be an asshole.
The ToU is pretty clear about what behaviors aren't allowed in League. If people can't be bothered to read and understand that, then I doubt they'd take the time to so much as blink in the direction of a comprehensive list of unwelcome behavior.
Maybe if they saw that they can't say kys you'd see less people wondering why they got banned and so on
They seriously need to be told that telling someone to kill themselves over a videogame is bad?
or they could just, idk, outright ban the terrible language in the first place.
Much like the idea of creating a comprehensive list of behaviors/words to avoid using, systematically preventing those words from ever being said is completely infeasible, simply because of how many different iterations and mispellings would have to be accounted for, for every word.
For example, let's look at the word "bitch". Ignoring having the letters in different cases, you have;
bitch, 8itch, b1tch, bi7ch, bitc#, 81tch, 817ch, 817t#, b1tc#, b17ch, b17c#, bi7c#, 8i7c#, 6itch, 61tch, 617ch, 617c#...
And it goes on. And this is just a couple variations accounting for a scant few replacements for b, i, t, and h; there's of course ! that could be used inplace of i, I3 instead of b, etc.
Now imagine doing that for every single word that you want to blacklist.
It is altogether more feasible and cost-effective to simply punish people for breaking the rules than to try and pre-emptively prevent the rules from being broken in the first place.