H:SE currently has a list of on topic/off topic advice in ask. (Curiously the "what not to ask here" is under the heading "What topics can I ask about?" rather than the "What topics should I not ask about?"
The current list is: It is not about:
Genealogy Asking for reference material Questions answered by a simple Google search or to be found in a Wikipedia page Predicting the future based on historical trends Mythology General (non-human) Prehistory Conspiracy Theories or Pseudo-science
I'd like to add one more item
Specifically of course I'm referring to Holocaust Denialism. Wikipedia defines Holocaust Denalism as:
Scholars use the term "denial" to describe the views and methodology of Holocaust deniers in order to distinguish them from legitimate historical revisionists, who challenge orthodox interpretations of history using established historical methodologies. Holocaust deniers generally do not accept the term denial as an appropriate description of their activities, and use the term revisionism instead. The methodologies of Holocaust deniers are often based on a predetermined conclusion that ignores overwhelming historical evidence to the contrary. wikipedia
That pretty well summarizes the distinction I'm trying to make. I'm happy to discuss challenges to historical orthodoxies based on historical methodologies, but I don't want to engage in discussion/debate with those who are arguing from a predetermined conclusion.
I believe that community moderation is sufficient to resolve any ambiguity. I'm more comfortable with a downvote/closevote if I can point to a clearly documented community standard that we don't want to engage denialists.