It seems strange that the community asking about History would stop people from talking about history. what if a historian finds a better or answer or a historical untruth/inconsistency? it seems strange that the medium for them to show their research/discovery is taken. I think posts should be un-archived to allow the fixing of previous answers, the addition of new answers, or proof of a better answer.
Posts are automatically archived by reddit after several months to free up processing space; it's not something we have any control over. Users still have the ability to edit archived posts though.
We absolutely agree with your concern; it's the reason we've never added an "Answered" flair, and will never remove questions that have been asked before. There's always something more to be added.
It is a site level setting we have no control over. All threads on reddit archive after 6 months. If you find content older than 6 months that have serious methodological issues, please reach out to us via modmail and we can remove if deemed warranted.