I remember hearing something like this, but if I recall it wasn't a specific ability to "play dead" but rather just that it's possible to knock zeds down (momentarily unconscious?) instead of them just standing upright while looking dizzy when flinched.
In a way, this can be a disadvantage to the players, as you may think you've killed a Zed and then turn your back on it.. Commandos (and anyone sharing their health-bar view, if the commando takes the requisite skill for that) will of course have an advantage here, as will anyone using excessive force or weapons that violently dismember enemies on death. (IE a berzerker can always assume that anything not in four separate pieces is probably still alive
)
In other ways, this can be an advantage for us:
An enemy that falls over can be
momentarily ignored while other threats are dealt with, and won't block view of still-capable enemies.
They may fall into a level hazard in the process.
Firebugs can get in the habit of playing "cleaner" and torching enemy bodies; If they aren't actually dead yet, you can probably inflict a good amount of burn damage by the time they get back on their feet. (and any corpse NOT blackened can be considered "not yet inspected")