This is just my perspective and what I have seen over the years. I could be totally off as a result of small sample size.
I've been playing since.....mid 2012? Whenever Darius came out. And the toxicity really got worse since then but recently it hasn't changed despite what Riot says. I'm guessing you have had the players who run to the enemy turret because they didn't get their role or you didn't go a certain champion, etc etc. I guess the issue is the self-entitlement where everyone thinks they are the playmaker and the team depends on them, so everyone should do as they say when they say it. And the moment you don't do something they don't like, well, now you know who your mother was sleeping with last night.
Look at how long it took Tyler1 to get banned. But then there are people who want him unbanned, despite how negative he can be if he doesn't get his way. But people would be pissed if he was on their team and was acting like that. I report when I need to and I don't get any notification if my reporting has done anything. So how am I to know that the guy who said he is a korean smurf and decided to run into the enemy turret 17 times to feed got punished?
Personally, I don't know where League is headed anymore. I started playing Overwatch and been busy with Pokemon Go (gotta live up to my name here) and now I have a hard time jumping into norms or even ranked as a result of the fear of feeders and toxic behavior and having to put up with it for 20-30 mins. Versus overwatch you can leave mid game and games usually last 10-15 minutes.