Verbal/negative attitude/language are the only reports that legitimately can be proven in League
It has occurred to me that the only real reports you can prove in League are reports based on what someone said in chat.
Let's face it, it is really hard to prove a report on a troll, unless they are being completely obvious. No one sees the games and no one could with the length of games and how many games are played. If a troll chooses to not use chat, there is no proof of what they are doing unless the game is being watched and it will make you look like the asshole for calling them out, even if it is legit.
Intentional Feeding is hard to prove unless it is obvious. I've had 2-20 games before, and I've had 0-12 games before. I wasn't feeding, just was bad. Teammates don't even know because you can't play the game and watch another person's lane. Unless someone buys something stupid like 6 pairs of tears or boots or purposely runs it down mid, you can't prove it.
Even Intentional AFK is hard to prove. Riot's remake system doesn't know when someone has left on purpose or simply just can't connect. Weather, internet connectivity and computer power are also issues that aren't intentional but a fact of life for a lot of League players. You can easily just jungle farm if you get tired of the game and wait for the enemy team to win and you can always just close the client and blame it on a computer issue. The only way you know is if someone outright says they will AFK.
Just something interesting to think of and kind of shows how easy it is to mask behaviors in League and why people are getting punished more than ever now for things said in chat - because it's the only thing you can prove.