It already works that way, OP just wants it to be told to players since… Most truly do have NO idea.
I do the same thing except I hide and pull out a book and read or will watch a YouTube vid on my phone until the game is over.
I got quit banned by 343 back when the game came out due to crashes and quitting when matchmaking would put me in 1v4s due to how broken it was, so now I refuse to quit under any circumstances.
No thanks. If I’m doing ok I don’t want to feel punished by losing xp just because some players can’t be bothered to stay in-game. 
Some uneven teams can be won if the opposing team is average/bad plus quitting means losing exp. All this means is I’m going to stay in-game find sneaky places to ambush opponents and take them out. As for the spawn killing I wish 343i cared enough to deal with these types of cheaters but they have no interest in dealing with those. If people tell me to quit for ‘dragging’ out the game then maybe they should quit instead. 
I was advocating for staying in a match, not leaving haha.
Why do all that when a simple window will do?
To end the match more elegantly than having individual players quit one-by-one, and to give the option to tie the game rather than give a free win for one team and mark a loss for the other.
A pop-up would be a good start for now, but I think we can refine the process.
As elegant as your solution is, it’s just a bit much. Who cares if the game ends in a tie? Even if they did most people I met in MM want to keep killing off the enemy. It’ll barely see use, so just have the window and be done with it.
I still think a surrender option would end the game more cleanly, but a window would go a long way to reassuring people that they can leave without penalty.
This is actually a thing? How has this not been communicated clearly ever?
It’s not a thing, I only learned this by viewing this thread and I’ve been playing since early summer. It’s never mentioned in-game that I have seen. Granted, as others above have said there are certain games where I’d stay and have a good chance of winning(Sword base and countdown vs handicap settings users in particular) but other than that I’d gladly quit and move on to the next game hoping for better randoms.
It was communicated in patch notes… instead of in-game… lul
please no window. Imagine you are already 3v4, and right when you are in a firefight with an enemy player and your on the brink of killing him of and another player your team quits (wich makes it 2v4 and lifts the quitban), then you will get a pop-op making you unable to kill the enemy and lets the enemy have a free kill…
That would be realy frustrating. I think it would be better if there would be a textline in your hud (that stays for the rest of the match) that tells you, you can quit banfree. Then you can just (in my example) kill of the enemy first and then decide to quit or not.
The pop-up doesn’t have to cover the screen or take control away from you. It could appear in the corner, or as you suggest, appear as text on-screen. Maybe something in the chat.
But then it’s not realy a pop-up isn’t it? A pop up is a 2nd window that opens up and has to be removed (by clicking X, or okay, etc) to return to the game.
If its a text notification in the hud, then its not a problem obviously. I also suggest making that text a different colour then blue to make it clearly visible then.
No dude he means like a popup window that appears from the bottom right corner, like the weekly challenges.
That said I don’t really care the situation you described happens. Yeah sure it’s an annoying way to cheat you out of a kill but if the match is 4v2 it was probably going to go poorly anyhow.
After having this same situation a few times over the last few days, I have to agree with the OP. No mention of this feature is ever given in-game. My brother and I had to wait out 3 games last night where two or three of the opponents quit early, yet at least one straggler remained with no hope of winning. Other than helping him progress towards the Domination achievement it was just a waste of time
A vague concern about hypothetical abuse is not a good reason to avoid adding a helpful feature.