This debate is about the D&D system, so of course I'm referring to game terms. The game needed some standardisation of terms, so TSR used some typical combinations as standard sets. As I said, I don't know how it's handled in later editions, but in AD&D 2e, you could combine individual pieces of armour to different outcomes and the available gear was at least modelled after historical examples. While Gambeson would be "Padded Armor", Mail, Brigandine and Plate were all terms commonly used in 2nd edition. This is however not to say that earlier editions were supposed to mirror reality in any way. But, the books usually gave you some historical context before saying, for example, how they had to nerf crossbows to keep them balanced for the game.
So, you're saying Wikipedia is no reliable source of information, but one YouTube video is? The steel plate cuirass in my example is a piece of armour that covers the upper torso front and back. It goes back to at least classical antiquity and was initially made of leather. Later they were also made of brass, bronze, cloth, iron and finally steel with modern composite versions, like bullet-proof vests still evolving.
While I can read and type bits and pieces, I don't usually have time to watch a whole 25-minute-video during my break, but the armour they're showing in the video is not a historical piece, but a modern replica which is made of steel. This was not necessarily the case with the originals, and if, we'd be back at the start of this tangent that this type of armour would be something owned by royalty, not used by foot soldiers. Nowhere I said earlier times are worse than later times, but in the portion of history relevant to most D&D settings, it was always a race between armour and projectile, technological progress still taking place today. Before the battle of Crécy, the mounted knight dominated battlefields. Afterwards, he was more at home in competitive sports. Quite similar to what we're currently seeing with expensive tanks being destroyed by cheap drones. So, take the mounted knight and the English longbow, the introduction of lighter but sturdier steel plates, the rifled barrel,...all these were game changers (no pun intended), but as a dungeon master, you will have to make the decision whether you allow firearms or which other equipment is or isn't available, as this will have a massive impact on the style of your campaign. Tech levels may be a game term, but science does something very similar when discussing technological development, for instance, the shift from the Neolithic to the Chalcolithic era. One civilisation could reach a more advanced state than its contemporaries, so while these terms give you a rough idea of the historical period they are also descriptions of available technology.
The reference to Dark Souls was because of the idiosyncratic rolling around. Badly fitting or heavy iron armour would not allow that, while even full steel suits were surprisingly functional for their perceived bulk if properly customised to the wearer. You're also right that there was a good chance an incompatible piece of armour could injure you badly.
