Give some form of compensation for AFKs.

I honestly don't care how you do it, but there is no reason players should lose upwards of 20 LP for a 4v5 loss. I understand that if you institute something against this problem, it's really exploitable, but there has to be some sort of solution to this issue. Basically, in order to climb, you have to win more 4full effort like a 5-man team would?

"Well, MajorTrump, you will run into just as many games where the enemy has an afk instead of your team". Technically true, but it evens out to half-ish. That doesn't mean it's fair.

Ok, so here's the solution. Recognize "Quitting" as opposed to "disconnecting". If someone quits a game, give them a flat 50 LP drop, then give the rest of the team an LP "forgiveness" of only losing like 15 flat. If someone has to leave for an emergency, then that shouldn't be a huge factor. They can make up that LP in a few games, which shouldn't be an issue if that emergency was a one-time thing. It discourages rage-quitting, but doesn't give enough benefit to be worth exploiting all that much. If you can put in place a system where you notice that teammates are quitting frequently in someone's games (exploitation of like minimum 1/2 the person's games for a streak of 5+), decrease LP gains on wins.

The system might be really complicated, but I think this really is an important part of a good matchmaking experience to be able to not be punished for someone else spoiling the game for you.