Destructible Demons

Destructible Demons is a game mechanic that was first implemented in Doom (2016), even though it wasn't named until Doom Eternal, where it was greatly expanded upon. It should also be noted that there is an unused giblet texture file in Doom.

It is a gore system that, for the most part, is purely cosmetic, but in key situations, allows for manipulation of the battlefield in the player's favour, in that it allows for the bodies of the demons to incur continuous physical damage from the player's attacks. This damage can include key spots on enemies' bodies which alter how they behave and can be approached.

Doom (2016)
In the 2016 game, the Destructible Demons system was not named yet and was, for the most part, a graphical system that created superficial damage to the enemies of the game. However, there were two notable demons that had specific gameplay alterations when certain parts of their bodies were targeted: The Cacodemon and the Pinky.

The Cacodemon's eye is a weak point. If enough direct damage is dealt to the eye, it will explode. The result is the Cacodemon unleashing a barrage of energy blasts in random directions before self-destructing. Though not likely to kill any of the heavier demons, the Cacodemon's barrage can certainly pitch in some extra chip damage in a group setting, though care should be taken to avoid being hit by the blasts.

The Pinky, on the other hand, has a pair of armour plates protecting its head from incoming damage. These armour plates can be blown off via the use of angled explosives (direct hits with the Rocket Launcher don't seem to suffice), such as explosive barrels, grenades, or the Rocket Launcher's Remote Detonation mod. When blown off, the Pinky's face becomes just as susceptible to damage as its rear. Due to the extra effort and relatively small payoff, its generally preferable to just dodge the Pinky and shoot its backside as one normally would. It's worth noting that the Spectre enemy present in the game does not have the armour plates on its face in gameplay, but does have them in the game's Model Viewer. Additionally, in Snapmap, Pinkies can be spawned with or without head armour, though oddly the default has them without it.

Doom Eternal
In the sequel, the Destructible Demons system was named and expanded upon. The superficial damage has been amplified to emphasize the brutality of the game's violence, with nearly every portion of the demons' bodies able to be stripped to the bone. In addition, more gameplay altering focal points have been added, while the two from the previous game have been removed for unknown reasons.

The Arachnotron possesses a turret mounted on top of it that can be destroyed either by continuous low-damage focused fire, or singular high-damage rounds. Because the turret can be very deadly in early-game, taking it out should be a top priority as it will deprive the Arachnotron of its primary source of ranged damage, forcing it to rely on its grenades and melee attacks. While its legs can be stripped to their frames, this does not effect the demon's mobility in any way.

The Revenant's shoulder-mounted rocket launchers can be blown off with enough damage. Doing so will radically alter their behavior, as the guns are the Rev's only method of ranged attack. Without them, the Revenants will charge after the player for direct melee attacks. They are still capable of using their jetpacks for mobility, however.

The Mancubus's cannons can be blown up similarly to the Arachtron's turret. Doing so will severely weaken the Manc's projectiles and completely disable its flamethrower attack. It will not prevent the Mancubus from being able to use the close-range AoE attack, however. Additionally, the Cyber-Mancubus returns and, unlike the sturdier 2016 version, this one is actually slightly squishier than than its default counterpart. However, the armour it wears protects it from incoming damage. As such, this armour can be blown off with enough damage of its own. The Cyber-Manc's cannons, however, cannot be destroyed.

The Doom Hunter enemy has a tank below its waist that serves as an artillery unit as well as a shield generator. The player can choose to overload its shield and damage the demon until it recharges, or can choose to destroy the unprotected tank, which will permanently disable the shields and missile launcher, while giving the Hunter greater mobility to compensate for the lack of offense and defense.

Lastly, the Maykr Drone and Blood Maykr enemies possess a very fragile weakpoint in the form of their heads. If the head is struck with a high-damage shot, it will explode, instantly killing the enemy, and releasing large amounts of ammo to pick up. The Maykr Drone's head is always exposed, while the Blood Maykr's head only becomes exposed when preparing a strong attack.

The introduction of the Spirit in the The Ancient Gods - Part One DLC brings with it new alterations to the DD system. When a demon is possessed by a Spirit, they gain a massive boost in defense and their bodies incur much less cosmetic damage from the player's attacks. Additionally, their weak points are made completely invulnerable to being destroyed.

Trivia

 * The Tyrant possesses a weak point on its arm cannon, however this was disabled prior to the game's release. If the weak point is restored via modding, destroying it will disable the Tyrant's laser and missile rain attacks, and reduce its rate of fire. This weakpoint was likely removed due to it almost completely nullifying the Tyrant's presence on the battlefield.