I agree, whenever you see 'a' historic in print, or hear someone say it, it gives the impression the person is poorly educated. While 'a' has become more common than 'an' over time, it's still something most well educated or well read people people don't use unless it's an artifact of their regional dialect. I live in a college town and rarely hear that form.
No. The use of "an" has become common, but "a" is perfectly correct. "An" is used before words that begin with a vowel sound. "Historic" begins with a consonant sound, "h". We don't say "an hippopotamus", "an horror movie", or "an hill". Using "an" before "historic" is a kind of hypercorrection, so I agree with Duolingo that it's incorrect. But I wouldn't argue if Duo chooses to accept it, because it's so common now. But "a historic" should absolutely be accepted.
For more discussion on the topic, do a web search on "hypercorrection an historic" or see, for example, https://painintheenglish.com/case/330.
Use “an” when the word following it starts with a vowel or an unsounded “h.” The Gregg Reference Manual, Tenth Edition (1101), says, “In speech, both “a historic occasion” and “an historic occasion” are okay. It depends on whether the “his” is sounded or left silent.Jan 6, 2011