This is a behavior that's so easy to detect, but Riot does the least about. Riot's policy is to give them the benefit of every excuse possible. "Well you wouldn't want to punish someone whose modem just happened to be struck my lightning, and then his replacement modem was ALSO struck my lightning the next game. So back to back leaver games may not be intentional." In reality, people leave games (1) knowing that their leaving was a risk when they queue'd (bad internet, competing real life priorities), or (2) ragequitters. The impact is -18 LP for 4 innocent bystanders.
Leavers should be punished properly. My most recent game, screenshot attached, is how blatant leavers have become. They don't care. They announce it in chat, because they know it'll never be punished. I played with a guy two days ago whose username was "I DC A LOT" (not his actual name, to protect the Board rules I'm not calling out an actual account ID, but it's very similar to that spirit). Sure enough he left 3 times during the match. It's such a joke, in the post game lobby I asked him about his name. He said "this is my account when I'm at school and know my internet will cut out a lot." I asked if he had been punished and he said "Not yet, and I disconnect all the time. Riot doesn't seem to track it, I'm thinking of just going back to my main since they don't enforce leaving."