Just because you're not getting notifications doesn't mean they aren't getting penalties or flags. One/two game doesn't represent their general overall every game. The IFS only detects consistency or severity. Everyone has a bad game, but even then they will still get a flag if it's legitimate.
Just know one game doesn't mean they're at their last flag strike before a penalty.
I understand you might think they deserve it before they ruined your one/two games you've experienced with them. But it doesn't mean they do it every game. But every flag counts. Building up over time before they get a penalty tier. I've been through it plenty of times, dealing with toxic players. But I do report them and I do not expect a notification. I am used to it and surprised if I ever do get one.
I just know Riot isn't liable to give me information on a player, regardless if they ruined my game or not. I just know I did my part by reporting them, and I know they will get a flag for it. I know what triggers as a legitimate report and I use it whenever necessary.
But remember, regardless what anyone says, just mute and report them.
Enjoy the game for what it is not for the people who's trying to ruin it. But don't let them ruin it with their toxicity. Just keep reporting them. They will eventually get punished, and you were there to help where it counted.