The info text for the Steam achievement "Who Needs Tygan?" reads "Beat the final mission using only conventional gear".
What exactly does conventional gear mean in that context?
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Sign up to join this communityApparently, coming from someone who did check in the game's files for the achievement conditions :
Of note is that this only applies to the final mission, you can happily upgrade throughout the game, just make sure your squad going into the final mission is downgraded. (Tip: Psionics/Dominate is AWESOME).
Only Tier 0 items:
- Assault Rifle, Pistol, Shotgun, Cannon, Sniper Rifle, Sword, Gremlin Mark I, Psi Amp, Grenade Launcher
- Nanofiber Vest
- Medikit
- Frag Grenade
- Flashbang
- Smoke Grenade
- PCS Upgrades
- Weapon Mods
Exceptionally, the following upgrades are allowed: (since you can't downgrade)
- Alien/Plasma Grenade (Permanent upgrade to Frag Grenade)
- Nano Medikit (Permanent upgrade to Medikit)
- Smoke Bomb (Permanent upgrade to Smoke Grenade)
Forbidden Items include (not comprehensive - ANYTHING not in the list above may be bad):
- Skulljack
- Battle Scanner, Mindshield, Mimic Beacon
- ANY Special Ammo (Talon, Venom, Dragon, AP, etc.)
- ANY Special Armor (EXO Suit, Warden armor etc.)
- ANY Special Grenade (Gas, EMP, Acid Grenade/Bomb, etc.)
- ANY better weapon (Advanced Grenade Launcher, Arc Blade, etc.)
In essence, the game checks the inventories of all soldiers and makes sure that you only have Tier 0 items, with the exception of the three permanent upgrades (Alien Grenade, NanoMedikit, Smoke Bomb) and any drops you got throughout the mission.
Interestingly, the Nanofiber Vest is allowed as well.
Source : XCOM Reddit