I know of the following rules for creating a proposal on Area 51:
- The scope must be reasonably well-defined, not too unclear (e.g. "cool stuff"), too opinion-based (e.g. "car reviews"), too broad (e.g. "life on Earth"), or too narrow (e.g. "The 1995 academic misconduct scandal at East Oak Mews High School").
- One must must must come to Area 51 with an already existing community rather than hope to build one on Area 51.
- Proposals that are subsets of existing sites are not acceptable (e.g. Discord API — deleted). These get closed as "This subject is already well-covered by a live Stack Exchange site. We generally do not split off subjects simply to give them their own space."
- Proposals for non-English sites are not currently being accepted.
Other than the criteria above, are there any per se restrictions on site content for an Area 51 proposal? If so, what topics are currently banned? I would expect a proposal for a "Let's organize a lawsuit against Stack Exchange" or "[Specific SE staff member] sucks, amirite?" would get closed for obvious reasons, but I'm interested if there are any non-obvious topics that are not currently allowed in proposals regardless of how well-defined, well-scoped, original, and well-supported by a community they are.
Having a list of banned topics would be helpful to potential proposers as well as the communities that back them. For example, I wouldn't want to go to all the effort of organizing a community, bringing them to Area 51, etc., only to have my proposal closed with a curt "Sorry, due to the political climate, we are not currently accepting proposals for sites about real-life weapons." custom closure.
To be clear, I am asking solely about topic restrictions (e.g. "no sites about fishing, hunting, or weapons"). I'm not asking about clarity requirements, the requirement to have a pre-existing community, not duplicating an existing site, or the requirement to not use sock puppets to artificially inflate proposal stats. Those requirements are already well-defined elsewhere or else are common sense.
Also to be clear, I don't have a specific proposal idea in mind right now or the community to make it happen. I'm asking in general. The question could be rephrased as "What are those topics where I shouldn't even bother to try to define, gather a community, etc. because any attempt to propose a site will be summarily closed due to covering a banned topic?"
This is not a duplicate of Can an Area 51 proposal be "expanded" by the community to include subjects diametrically opposed to the original proposal? because that question is about the appropriateness of expanding a proposal to cover things that are acceptable proposal topics but arguably don't make sense in combination with the original proposal scope. My question is about topics that are completely banned from proposals regardless of whether they are included in the original proposal description or tacked on by the community later.