I actually posted about this 5 years ago: https://support.khanacademy.org/hc/en-us/community/posts/360056520172-Adding-Unanswered-option-to-Community-Questions-listing; the post was closed 2 years ago.
It was also suggested 6 years before that (so ~2014): https://khanacademy.zendesk.com/hc/en-us/community/posts/201310134-Introduce-Answered-Unanswered-All-for-all-community-questions [the link does not work]

Until the option is available (if it ever is), what I sometimes do as a workaround is to browse the questions on the module (video/article) page, and just keep paging down to see if there are any unanswered questions from the past.  (While I do that, I often find many posts to flag -- if they get removed, there are fewer inappropriate posts in the Questions queue afterward. There does seem to be a limit on how many flags I can set in a time period, though.)