Abandonment penalties need to be added in all modes. I'm so tired of getting to second place as a duo and losing to a trio. It honestly happens a lot for some reason and it drives me nuts.
I just played a match where we were top 5 and right next to a beacon, and the guy quits as soon as he goes down (the other person and myself still won the fight). Second place finish as usual.
Maybe they should rethink what the penalty should be for leaving. I'm guessing they'd never put leavers into queues with other leavers, since they can barely handle matchmaking fair teams as is haha, but maybe there is some other way to penalize people? Maybe disable custom skins for an hour or something, I don't know.
My guess is the reason that they don't add abandonment penalties across the board (and most games don't) is that it likely increases your games churn rate.
If you think about the reasons people quit, it's often because they're frustrated about something that happened. Preventing them from quitting or punishes them for doing so likely makes them more annoyed and dissatisfied.
A penalty such as a timeout is likely to cause players to close the app and play something else. Ultimately that's not what you want as that risks players finding something they find more engaging in that time. Whereas preventing people from quitting altogether can create a lot of problems and exacerbate certain frustrations. For instance, my friends and I regularly quit a match if our friend is in the lobby waiting and our game has just started, or if the match is especially laggy out of the gate. Being forced to play those games is likely to increase the odds of people leaving.
You could argue that having people stay improves the overall experience for all players and therefore is likely to decrease churn. I think that's intuitive to assume but I don't think that's likely. If you make it so that people are forced to play they will often find ways to circumvent that system (e.g. by going afk or finding other ways to quit such as closing the game) or they'll stay in the game and behave in a way that's toxic to other players (e.g. instead of quitting because someone took your character, grief them for 15minutes).
There's not really a good option to solve this problem. The alternative is incentivising them to stay, but then it's not clear what kind of reward you could give for just staying in a game that wouldn't just become the norm anyway. You can adjust things like messaging and the overall UI flow of the quit to potentially make players reconsider but those approaches have quite limited effects.
Ranked gets to justify it's penalties because it really wouldn't function without a quitters penalty, and I think even those that want to quit are more understanding of why those penalties exist. Still, we've had 10 seasons of people finding exploits to quit ranked games and avoid loses. Then you have to start enforcing punishments to people that circumvent your systems (which they do, in ranked). But you also have some false positives when you do that... some people get banned just because their internet disconnects them too many times and whatnot.
If Apex had the same ruleset for quitting across the board, I would have already quit playing this game personally. In season 5 I used to have my game crash at least twice per day (due to a bug with a large friend list on PS4) and when I played ranked I would get a 15 minute penalty each time. That in itself was frustrating but the penalty would also double each time, so 15 minutes, then 30, then 60, then 120 etc, that's how the penalties work in Apex Legends. I was up at 6 hours or something like that and I decided to stop playing ranked.