CS took a little over a year in a half to do this, hopefully it takes you before release.
if somebody leaves during Lobby or Warm Up, give him the cooldown right there and cancel the game. Don’t start it 4v5 and let the game finish (Because you can finish a game in less than the time it takes for the cooldown to activate.)
And give an option to the player who “left” to abandon the game so if he’s not going to join back and didn’t leave by mistake, people aren’t wasting their time playing. The only people you are punishing by not allowing someone to just quit are the people stuck playing an unfair game, not the guy who quit.