After reading all of the posts on that link it appears they all have different opinions. I don't feel like it is evidence of slings being good or bad for hunting.
I think the problem is that people just don't do a lot of hunting with slings.
In modern times people use slings for fun because we have guns and modern bows for hunting.
Ok this answers many questions.
And yes i oversimplyfie this a little bit but i see this from the perspective of a player and in this case its possible that i have bend the reality a little bit to get a solution for this problem.
I think we must wait what the game will bring to see if a sling make any sense for hunting an Animal. I dont believe that Animals who can smell you come to the Game (this where very cool but i dont believe this) and from this point of view i ask myself how to make it possible to bring such items like a Sling or a Blowpipe (or anything like this) to the Game without beeing to unrealistic.
I think there should still be a small chance for zeds to spawn outside their designated areas. May not be necessary if they are able to break out and wander on their own, though. It's been a while I can't recall if that can happen off-screen.
Seeing a zombie walking down the street in a bathrobe once in a blue moon would be cool.
Or seeing a zed in a spiffo costume at the police station and wondering why he was arrested.
You're oversimplifying it as if you could hit a deer in the head at 100mph (quite the feat) with a sling just with some practice and a dream. It's not an effective way to kill big game regardless of how good you get at it. Especially if the deer is moving, or any considerable distance away from you (and the getting close to one without it being aware of you with any of it's viable senses - especially smell - is challenge enough).
Not to mention the fact that if you stood as close as the man in that video, the winding up of you starting to swing that for momentum will scare it away instantly.
And there's a fairly obvious reason hunters don't use poisons to kill the animals that they eat. Regardless of what you hear or read about tradition, that poison will be in the meat. There have been cases where people were charged with killing protected animals because those animals died eating the flesh of other creatures that they poisoned. It's not recommended.
As said above, slings are useful for other reasons. Just not hunting deer. Or much else that has evolved and adapted to perceiving and outrunning fast game. The first obstacle is getting close to one. If it isn't socialized to humans (as most surviving deer in this scenario would definitely not be) this often involves masking your own scent and camouflage, and often knowledge that people don't just suddenly pick up. If you are trying to be practical, there are already much better ways to hunt that don't need to be reinvented.
I went on a slinging forum and found this exact question, and they disagree as well. At most, some claim to kill small game, but inconsistently. Keep in mind that they are talking about small game.
Sorry. It's a fun idea, but for hunting deer, it just doesn't sound like this is going to work. There are other reasons for slings though.