Ion explicited stated so when asked previously on why there isn't a leaver mechanic/penalty in place. His words were that there wasn't a system that couldn't be abused in such a way that allowed a minority (aka 1 player) to hold up other players as hostage. His recommendation was that M+ should be done with friends/guildmates. Let Ion speak for himself go look at the link.
Because evidence is key look at Ion's Q&A for January 24th, 2019 (wowhead link)
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I've seen it happen plenty of times in LFR/LFD. I've heard stories about it from Island Expeditions (because the threshold is lower, only 2 votes needed to kick the third).
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Ok so in that kind of system, what prevents a player from messing up so much that you want to leave but can't because if you leave, you're the one stuck with all those penalties?
Or let's say it's a group that somehow got a hold of a really high key that there's no way they could do it but you didn't find out that they couldn't until after they started the run. Now you're stuck in a terri-bad group that has ZERO chance of finishing the key in time. At best you estimate that it will take at least an hour if not more. Do you HS out?
Alternatively what if players don't HS out but instead just log out? Do they get penalized?
Or what if, a player does go offline but it's because their power/internet went out and there's nothing they could do about it. Should they also be penalized? After all they did "quit" even if it wasn't intentional.