Do you get penalized for leaving seconds before the game ends?

Not sure if this was already answered. I saw a message for afk right after the game ended but it was very quick and didn't even stick (couldn't find it) and didn't penalize me so it was prob nothing. It did get me thinking that the last few games I would alt f4 twice out the game when the nexus blows out (you hear the animation). Pretty sure this was fine since there weren't any timers like you normally do when exiting the game in the middle. But do you get penalized for it? It's just a quicker way to exit and having "defeat" in your face isn't really a great feeling, especially when you're on a losing streak.