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The term, at least on bomb suits, is usually "neck dam" or "collar extension", but may also be called "high collar" or "collar flap".
While I couldn't find any existing trope, and like you said, a lot of these have probably taken some inspiration from real-world bomb suits (EOD armor). Check out what the guy is wearing in the Wikipedia article for Advanced Bomb Suit
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These frequently draw from exaggerations of traditional plate armor:
Both Eastern and Western versions of the cuirass frequently included a high collar, which sometimes got exaggerated to High Collar of Doom or Shoulders of Doom levels in the interest of minimizing the amount of access pointy objects had to the neck.
Late in the era of plate armor when smiths really got good at creating complicated mechanical joints you also had a lot of variants of the bevor and buffe, two styles of neck armor that would get matched up with a helmet so that they overlapped or interlocked to create your bullet-head profile
Having looked at No-Neck Chump and Tiny-Headed Behemoth it seems like there isnt really an existing trope for this, since Shoulders of Doom applies to shoulders and High Collar of Doom applies exclusively to villains (and isnt specific to making them look durable so much as imposing.
I wonder if there are enough examples to justify a new trope...
Especially since in real life the tendency for modern armor to include a neck dam over a gorget collar is still fairly uncommon due to weight.
Some more examples Ive thought of:
Davis and the Holo Pilot class in Titanfall 2
Fortified Commando
and dreadnought
and Devastator
armors from Helldivers 2
Maestro
and fenris
◊ from Rainbow Six Siege
Spooky Space Kook
from Scooby Doo
Terminators
and XV25 Stealthsuits
and Riptide Crisis Suits
and Stormsurge Battlesuits
from Warhammer 40,000...

what's the term for where, in order to make an individual appear more armored (regardless of how armored they actually are) their armor will make it look like they don't have a neck (so their body goes straight from chest to head) or don't have a head or neck at all.
Some examples:
Ultimate Juggernaut
exaggerates the design of regular 616 Marvel Juggernaut, still giving him the appearance of being heavily armored while wearing less armor than basically about as much armor as Conan the Barbarian.
Tokugawa
from Lancer
The MCU version of the Hulkbuster
◊
Armored Trooper VOTOMS
◊ (for the unaware the pilot's head is actually in the head of this mech, VOTOMS are really small, that chest is basically entirely cockpit with the pilot seated inside it in a pressure suit)
Juggernaut armor from Call of Duty
and its real world inspiration EOD armor
Note that this is different from a gorget, in that it's not armor that conforms to the neck, but rather armor that obscures the neck entirely, often giving a character the appearance of a bullet head, box head, or no head at all.