Faolyn
(she/her)
Anyone can cast in armor if they are proficient in the armor. The Eldritch Knight is no different than a wizard with proficiency in armor. Which is why there was so a to-do about letting races have floating ASIs; it mean that mountain dwarfs could put their +2 in Int and wear armor at the same time.The Fighter is proficient with "ALL" armor. An Eldritch Knight can spellcast in "ALL" armor.
The situation is different from the Druid.
The Druid lacks proficiency in metal armor therefore cannot spellcast in it.
And again, all armor proficiencies are divided into "light armor," "medium armor," and "heavy armor." And shields. There are no materials-based armor proficiencies.