It's "they were hurt." "You" used to be second person plural pronoun with "thou" as the second person singular pronoun. When "you" replaced "thou" as the second person singular pronoun as well, the conjugation of verbs didn't change. So, most English dialects wouldn't say "You is tall." or "You eats too much food" but "You are tall" or "You eat too much food." It's the original definition of "you" being the second person plural that verbs get conjugated as if "you" still has that plural meaning. I don't see why this principle wouldn't extend to "they" as well.
It's "they were hurt." "You" used to be second person plural pronoun with "thou" as the second person singular pronoun. When "you" replaced "thou" as the second person singular pronoun as well, the conjugation of verbs didn't change. So, most English dialects wouldn't say "You is tall." or "You eats too much food" but "You are tall" or "You eat too much food." It's the original definition of "you" being the second person plural that verbs get conjugated as if "you" still has that plural meaning. I don't see why this principle wouldn't extend to "they" as well.