I keep hearing they were removed, but I don't think we've actually seen any boss footage other than what was shown couple years ago. If I remember correctly, it was the Siegebreaker Assault Beast chomping on your character's head.
If the reasoning for removing them was "we don't want to create an incentive to let your character die", then I guess I can understand it, but I still think the concept is badass.
EDIT: This is the kill I'm talking about: http://youtu.be/iWGL_DthwX0
EDIT 2: Better video, lol