For the last 2-3 years, you can have as many accounts as you wish linked to the same email. Even if it was 1 account per email like it has been for the first years of LoL, it takes a few seconds to create an email. Email blocking wont mitigate the issue enough.
IP ban is not even efficient because any VPN can change your IP, or reset your router, and it would be unfair if you live under the same roof as a toxic player but you are not toxic yourself.
ID ban requires too much human resources for tracking and watching streams.
The strongest way would be ID verification like in Korea with Social Security number but it can also be abused with buying account or using your grandma's ID. Also, some players from poorer countries with less developed identification archiving and tracking would be put at disadvantage.
Another angle is to destroy the botting business. Because i think lots of permabanned players dont have the courage to level up a brand new account by themselves. I think destroying the botting business will mechanically discourage chronic toxic players.
The permanent ban is the current final decision. I dont have numbers but the goal is to have a permabanned player uninstall the game. You have to be seriously addicted in an unhealthy way to keep creating accounts and being toxic again and again.