Is it reportable to be toxic in league messages?

League message are considered to be an opt-in experience, and as such are not directly reportable. That said, certain severe behaviors can be sent to Player Support, who may look into players who are adding others for the sole purpose of being toxic while evading the system.

There is no reason to ask this unless you are looking for ways to be toxic to people that won't get you in trouble.... or if someone did this to you and you want to know if THEY should be in trouble.

If the former, you're a jerk and you SHOULD be reported and banned. If the later... no. They're trying to be dicks and avoid reprisal. I'd suggest blocking them immediately and if they were really bad, screenshoting whatever toxic crap they hit you with and sending it to Riot as a support ticket. I'd also suggest not accepting friend requests from people you were just in a game with unless they were actually being nice to you during game or something. You'd be shocked how many people will friend you just to be toxic AF in a way they think can't get them punished. Jokes on them, they can TOTALLY be punished for it.