What everyone here is circling around in saying is go send a ticket or seek the help of Live Chat at https://support.riotgames.com/hc/en-us/requests/new ! Perhaps if you throw yourself at the mercy of the court and let them know the repeated sayings were in error they coooould forgive you for the first.
There actually is precedence for a ban initiated for saying KYS to be overturned and is displayed in this forum if you scroll enough, as it was looked at in direct context of how It was used (in this instance, they may have been cheering a Sion or something to go yolo on the enemy nexus to finish the game).
Well, a banned account doesn't mean you can't have a new leaf in season 9. In fact, it is the chance to have THE NEWEST LEAF for you! To be the face of a "new player' who says nothing but nice, kind, positive, friendly, and encouraging things every game no matter what. What you miss out is only the Skins and things left behind, but do not give up hope! Join some League of Legend discord communities to enter contests for free RP! Make wonderful new friends who might feel like gifting you a skin for your great attitude! Maybe you end up in a match with a Riot staff who says "wow, this person has just the best attitude, i'ma give them a free skin for it." ---> (Hugs my DJ Sona skin <3)
Fresh Account also means Fresh placements! Might end up in Gold or higher on the first try! YOU CAN DO IT WE BELIEVE IN YOU! :D
But at the very least, just don't say KYS anymore, ever. There is no reason, its hurtful, and mean. Be a bigger person than that.