Riot, if you really want to tackle the immense toxicity in this community, you need to take a stand
AGAINST SMURFING.
Smurfs are like abusive parents raising children. New players come into LoL, and are met with a near constant stream of abusive smurfs that eventually either force those players to just leave the game entirely, or adopt abusive attitudes themselves, thereby continuing the vicious cycle of negativity and toxicity that is so rampant in this community.
It doesn't even need to be over the top verbal abuse, or in fact verbal at all. Can you imagine anything more frustrating in a game like this than to be put up against an opponent that you literally have 0 chance against? An opponent so far beyond your skill level that no matter how well you think youre playing, you can never come close to beating? An opponent who single handedly carries his team to victory simply because he's lightyears better than everyone else in the game? Such a scenario can and will instill a negative attitude in a player that will fester and grow game after game until they finally reach level 30 and start bringing that negative attitude into ranked games. In all likelihood they themselves will just make a smurf account and go "beat up" on newbies thus continuing this vicious cycle.
But this issue goes even deeper than just creating a cycle of toxicity in this community; It also breeds bad players. Its immeasurably difficult to really glean any real knowledge from a defeat by an opponent so ridiculously better than you. Factor in the negative attitude the environment forces upon a player and you have a scenario in which it is HIGHLY LIKELY that a player will reach max level with very limited game knowledge or how to properly play the game because they are so used to just getting annihilated by better players. This creates an endless supply of low elo players who likely have an extremely toxic attitude and creates an "elo hell" in the lower tiers of the game. Sure, eventually some of these players will manage to climb higher after thousands upon thousands of games, but their attitudes remain the same except now they'll have an heir of entitlement on top of their negativity.
All of this could be avoided if Riot just took a firm stance against smurfing, ESPECIALLY against toxic smurfs.
If you want to try to fix the toxicity in the community, you need to start from the ground up, and stop the creation of new toxic members of the community from being created.