As far as I can recall, there has never been anything that instantly kills you when playing in squads or duos. I always go down if I have a teammate. Of course in solos, it's always an insta-kill. The only times you instantly die in duos/squads is if you get knocked out while swimming/in a boat because your character goes underwater and you cannot get revived while underwater.
According to the Wiki page for Down but Not Out, on your 7th knock down, you have 0 seconds to get revived, which would indicate an instant death.
- You can enter the DBNO state multiple times during a match, but the time allowed for you to be revived will reduce each subsequent time (90s > 33s > 20s > 14s > 11s > 9s > 0s).
I can't say I've ever played a match where I've been knocked 7 times. I most likely was killed way before that.
I suppose another way you can instantly die is if you are the last standing teammate on your team while the rest of your squad are all downed. If you take enough damage to go down, you will instead die instantly, as will the rest of your team since it is physically impossible to get revived when all of your teammates are down.