I'm not much of a source expert but IIRC it had the ability to create the ragdolls in a pose (Didn't work in older version of gmod because some stuff was broken, but the combine in older versions of HL2 used it all the time when killed) and then L4D and a TF2 update added the ability to play a animation before switching/fading to a ragdoll (See: Zombie stumble deaths and TF2 backstabs/headshots)
Yeah, but the thing is, i haven't seen these animations at all, Gmod Tower makes use of the gestures like dancing, waving, etc, so maybe they're gonna include the death animations in some way, but otherwise, i haven't seen them