Technology has to be available for it to be used. Even presuming the knowledge, constraints are cost, material, skilled labor, and time required. Plate armor was good against bullets but nobody wanted to wear it or pay for it with the advent of a focus on mobility and larger armies. People with expensive gear require servants and extra horses and such. Also cultural norms and concepts of battle dictate design. So pretty much any costume could be justified within a fictional setting. Also armor you wear for a portrait, taking your imaginary characters as real persons being portrayed by an artist, might be significantly different from what they actually wear.



But of course the primary deviant constraint on armor appears to be sexual appeal, with uniqueness also being a positive instead of a negative, since uniqueness is generally an added cost for a person actually designing and making something new.



This woman has etched armor, an etched blade, and an elaborate coiffure. The only quibble I see is that maybe -- since her armor seems to be steel, leather, and padding -- her width dimensions are not commensurate with the materials being used. In other words, her dimensions seem appropriate to normal clothing, not armor, since you have to allow for a body under whatever is worn.