IIRC, mongolian mounted archers wore no armor opting instead for speed and agility.

Do battlecruisers count?

Maybe those medieval militaries that relied heavily on foot archers, which were generally commoners/peasants who were very lightly armored? e.g. the English. The rest of their armies were more standard, but when armored enemy melee fighters/riders broke through the line, those archers were proper fucked.

Arguably most WW1 armies. They figured out early how to defend against armies of the era, but not how to beat those defenses and thus the war dragged on for years. Fortified stationary machine guns are devastating defensive weapons but you're not conquering anything with that.