Stuff happens, so not all cases of AFK are necessarily the player's fault (weather causing connection issues, parent's cutting the internet because of a "sudden" change of plans for the player as their child, Etc.), but it's... not really much if going afk gets you a loss of LP as if you had already lost the game, and so long as you don't do it too often, you won't really get Busted either.
So, something similar to what CSGO does, or so it is believed because there's no hard proof, that when a player leaves and is replaced with a bot, your team's MMR drops so winning with fewer players rewards those sticking it out. If we apply this to League (not the bot, the MMR drop thing), when a player leaves a game such that they receive a Leaverbuster Flag (not necessarily the penalty itself), they receive no winning LP, and receive up to twice the LP loss. Loss penalty is distributed evenly to other players as a loss _reduction_ (leaver receives a 4X LP penalty, other players receive an X LP loss reduction). Wins with a leaver do not award extra LP to the other players. If leaving during a promotional series, a win or remake gives one loss, and on a game loss, they receive two losses. Other players may receive LP compensation if they lose their promos as a result of a leaver game (multiple awards may be given at a reduced amount beyond the first).
Now, if a leaving player returns for an adequate amount of time, and hasn't been gone for too long (let's say AFK time is less than half the game, but they still received a Leaverbuster flag), they will receive less of an LP gain on a win, probably half, and they will only take half the extra penalty for losing. They may still receive wins during promos, and will not receive extra losses; other players as before will receive a reduced LP loss, but at a lower amount due to a lower penalty being issued to the leaver.
If a game has multiple leavers, the effects will stack accordingly. All LP loss reductions will be distributed evenly to non-leavers only, and with enough leavers may result in no LP loss to the non-leavers; the leavers then accumulating the total team LP loss evenly distributed among themselves.
What this does is incentivizes players to play long enough for the leaver to get busted and award them with a loss reduction, at which point they may also find the game winnable and actually win. Numbers are debatable, but the goal is to penalize leaving more, and to penalize those with leavers less, with some leniency if the leaver is trying to return.