Why is there not implemented a function where you can either report og mute someone while in champ select?
I'm not a big fan of the idea of pre-game chat mutes, only because (more often than not) people will flame after the game has started rather than beforehand. Muting someone prematurely also means you're losing out on valuable conversation such as ban discussion, tactics, or trades. IMO it would be more of a detriment than a benefit in most scenarios.
Can't really say much for post-game chat. I think in-game mutes should carry over.
As for the actual reason, I think it's because in-game and in-client chats take place on different systems. They probably just haven't added (or figured out a way to add) the ability to mute people across multiple stages because of that. Just speculation tho :p
The person is intentionally feeding every single game (I dodged) and kept telling everyone to kill themselves..
Just in case you remember their name, got a screenshot, or for the future:
If someone dodges but you manage to get the above information, you can attach that with a support ticket. Riot won't disclose the outcome, but if it's rulebreaking behavior it's a safe bet to assume they'll be punished.
Edit: Also, thank you Imperial Pandaa for reminding me of this:
OP: I'm pretty sure you aren't willing to jump into a game with someone you know is going to be toxic, but just incase you're willing to bite the bullet or don't really care: In-client reports made at the end of the game do cover pre-game chat, post-game chat, and everything inbetween.