Unbanning older "permabanned" LOL accounts would decrease toxicity, which is why Rito will not make it a thing, at least not beyond the few cases on NA. Before I explain why, here are some facts.
What do toxic permabanned players do? They buy smurfs which makes it an open season for toxicity, I know that from exp because I got around 15 accounts permad from 2012-2016. I'm a permaban professesional. I had an account that I spent like 300-400 euros on in 2015, which I got a well deserved 2 week ban on, then told myself that I'd stop being toxic, which I was, but not according to riot where "typing is toxic".
After that ban, I went John Wick mode, full hate for Riot, buying loads of smurfs (more than 10) and used them to see how I could game the instant feedback system, my mission was simple, be as toxic as I could possibly be in ranked games and see how far I could take it before another perma. After 2 years though, that stopped being fun too.
Ever since 2017 I just played 1 botgame every day, for the experience and to somewhat be up to date with the changes.
Anyway, unbanning old permabanned accounts would definitely not cause there to be more toxicity, league will always be toxic and there's nothing that will stop it. But people are 11 trillion times more less likely to be toxic on a "thin ice" unbanned account than on a new or purchased lol account. If I got the chance to get my old account back, I would deffo not waste it again by writing "please never play pc games again you %%%%%%ed oxygen thief" when losing a ranked game. If I wanted to do that, I'd do it on my useless fresh accounts that have nothing invested in them.
If riot wants to reduce toxicity, at least "temporarily" before it goes back to normal toxcity, unban all league accounts 2013-2016 for example. If people are stupid enough to use chat that is considered toxic by the ISF system, they dun goofed.