Here's the problem with your argument: the concept of 'toxic banter'.
I've seen banter in-game. I've definitely seen toxicity. Maybe there was a time when I would apply both terms to the same situation, but I can't think of one. Banter is, by definition, people talking back and forth; toxicity is pretty much always asshats yelling at people. Now, it's very possible two asshats will yell at each other, but that's still not banter.
I genuinely envy the fact you can enjoy any of that. But it becomes tiresome very quickly when you're trying to improve and that's how people react to it, especially when you play unranked games specifically because you want to be better before you play ranked. You are only speaking for yourself; Riot has to take the whole community into account, and it should be clear by now that enough of the community is driven away by this behavior (if basic human decency wasn't enough) for them to work on stamping it out.