Lets say I'm asking a question that simply cannot receive an answer aside from "We don't know". In this case, would this thread remain unaswered until it fades(thus possibly leading to reposts) or is it within the rules for flaired users to declare that there's no way to answer this question?
This is unanswerable. Sorry.
No, kidding!
Most of the time questions that are truly unanswerable fall on the bad side of the rules and are removed. Honestly, unanswerable questions are the very reason for the rules against throughout history and poll type questions. The ones that remain and cannot be answered either because we don't have an expert (see a lot of the Indian and Modern American questions) usually manage to attract a lot of one line answers and comments like "where are all the comments" that get removed.
There have been a few questions that haven't been answerable that the Flairs have put their own spin on like "well in this answer I'm going to assume you're talking about racist British porn instead of all black porn in the history of ever when I answer this..."
It's not against the rules for a flair (or anyone) to say the question is unanswerable as long as they explain why it is so in their answer. A good answer to an unanswerable question contextualizes it within history and so on. For example : "this is unanswerable because we don't have records of the sex toys of early Japanese people but based on later developments we might be able to say x y and z"
Hope that answers your question.
As a prehistorian I'd say 9/10 of my answers in this subreddit boil to some variation of "we don't know". I don't think that precludes them being in-depth and comprehensive, though. It's almost always possible to write a great deal about why we don't know something, what the spectrum of possible answers looks like, and/or what our prospects for ever being able to answer the question are.
For example, I've managed to churn out loads of verbiage saying:
Personally I think the best questions are unanswerable. That's why I got into prehistory. Who the hell wants to spend their time studying things that can be known?