If you're giving warning, you shouldn't be playing. My point being, you're not giving warning for the completely unexpected, you're giving warning for the somewhat possibly expected. It's incredibly selfish to make other people play through an uneven game because you chose to play when you didn't have time.
If you were to say this in champion select, and offer to dodge yourself if anyone is opposed fine. But to say you tell people doesn't mean anything - maybe I just dodged a troll and don't want to dodge again? Or if you're saying it in game, that's WAY too late. At that point I'm already 100% in.
Real life does happen. If something comes up that you had no idea could happen, leave. You'll never get banned if you only leave in these once-per-year or less instances.
When you queue for a game when you're on-call for work? That's your own fault. Queue when you know you need to pick up your kids in 55 min and the game goes that long? Your fault. Queue when your building has a planned power outage during an 8 hour window that day? Guess what, your fault. If you consistently do these types of things, you should get reported and should get banned.
I think the point comes down to this:
Leaving a game as an individual comes down to the decision "would I rather finish this, or do that". Let's normalize and say the game is worth 10 to every player, so if anything worth 11 or more to you comes up, you'll leave the game. The problem is, the other 4 teammates, or possibly 9 players, lose 10 and don't get that 11+ you got. The ideal situation would be for players to only leave for events worth 50 or more, so your individual gain outweighs everyone's losses. this is enforced by putting extra penalty on leavers - if you know you'll be banned from playing if you leave, you may only leave for events worth say 30+, meaning all of those 11-29 events that people used to leave for are eliminated at least.
Be considerate. That's what it comes down to. Is whatever you're leaving for worth making 4 other people suffer? Honestly, if it takes even a moment to actually have to consider whether to leave or not, you shouldn't be leaving. and if anything that could make you leave is reasonably expected to happen within 80 minutes, don't queue.