It can be annoying having the thought that one constantly abusive player on your team wont shut up even if you muted them it can leave negative feelings towards that person possibly resulting some people not cooperating with that person.
I've seen supports go top lane because of a flaming adc gets ks'd and spends more time flaming than farming free cs and pushing for a free tower.
Blind pick if someone does beat you to a role and you have no choice but to do another some players get really cocky and start talking shit to you in the lobby.
even if the player is muted just the presence of that jerk on someones team can lead to them performing worse because of negative emotions.
One jungler i had refused to gank any lane except my top lane because premade bot told him to get cancer.
Reason he came top often was because i was being nice to him/her.
Honestly if anyone flames and this causes someone to troll or afk that team because someone is being toxic constantly even if they are muted i can't blame em and this will deliver sweet karma to that jerk who deserved it.
as for the other 3 then thats too bad i have to deal with someone rage quitting on my team over any reason and yeah i wish they didn't do this but for goodness sake people be freaking nice.
I even have the friendly honor ribbon which sometimes switches to Honorable opponent ribbon.
I have been flamed by jerks like these before and it has hindered my performance, you dont' have time to shake the negative feelings off of you.
Why don't you try standing up for others and make em feel better even if they get flamed or suck at the game.
I'm not an adc main here man and supports always say im sorry if they ks or if i die in a fight.
I'm like "man dont worry its normals and plus I'm one of the nice players and honestly i can't take league seriously". Sometimes we go into a series of stupid jokes that are lewd, timely jokes about current events, or we do just silly shit while still playing the game up to par.
if anyone on EUW wants to play for fun come add me. feel free to build any silly thing on a champion me and some of my friends are on league from time to time and we sometimes do stupid team comps
5 team supports
Kled and the marathon runners (karma, singed, hecarim, sion.)
Or if we do One for all mode we play something stupid like 5 volibears and go like this https://www.youtube.com/watch?v=sASfLQm9iCc
TLDR: Yes abusive players are annoying, can hinder player performance even if they are muted. Be nice and stand up for others for once and you will get some good respect back in return. You should try that some time. Have honorable ribbon as proof.