I'd like to start this by saying I'm not a big fan of fiats from on-high. In general, I think the best way to resolve differences of opinion on SE sites is always going to be via votes on questions.
That being said, I'm afraid the issue of exactly what disciplines are considered on-topic on this stack is so fundamental that we simply cannot leave it up in the air. It causes confusion in new posters, and repetitive comment discussions and close/open fights. For instance, on this recent question, where a comment asking about how the user's question could be improved led to yet another rehash of this meta discussion in the comments (and a very confused user). The question was subsequently closed (likely by anti-prehistory people), and then reopened (likely by pro-prehistory people).
I believe we have come to the point where we need to make it clear, preferably on our help center topicality page, whether questions that necessarily must rely on non-written sources like DNA, linguistics, and oral history, are on-topic. The current situation of confusion is not good for the stack.
So what I'd like to do here is entertain ideas for the form of the modifications to our help center page. If you'd prefer to argue prehistory pro or con, that might be better carried out on the existing meta discussion page. I'd point out the current vote tally there is 5-1 pro-prehistory. However, that's literally 5 votes to 1. It really could use more votes.