You know... I spent over 2 years explaining this exact type of thing with detailed EVIDENCE and got nothing but grief from the boards.
I could still be playing despite having gotten chat restricted a couple times for losing my temper over seeing that type of crap over and over and nothing being done about it, but I'm done with it.
I applaud anyone who will still fight for the idea that Riot needs to step up to the plate and actually start dealing with griefing instead of excusing it. I'm happy to see that for once the boards are upvoting such a post rather than spam downvoting and writing mindless stupid replies that show they didn't even bother reading half of what you wrote like I've faced for over 2 years campaigning to fix this issue.
But until I see meaningful improvement in the way they treat players and the way they deal with griefing, I refuse to play anymore. Sad because I just found an irl friend who actually plays lol about a year ago, but playing the game even just for fun with them suggest that I support their horrible lazy policy in regards to griefing and their overly harsh policy towards "toxicity" which can be muted. Funny how toxicity of griefing which cannot be muted is not only ignored, but outright defended and excused by Riot while anything explicit in chat gets brought to the gallows even if it was deserved based on what was done and said to the player who snapped from being repeatedly abused both in chat and by gameplay such as your griefing teammates.
Best of luck to you, but I don't see anything changing any time soon.