Let's apply the current guidelines:
- If it's marketed as SF, it's on-topic.
Depends on the work, not automatically on-topic.
- If magic, futuristic science or technology, alternate history, or other sf-nal concept is an important part of the overall plot, it's on-topic. (Alice in Wonderland, Clockwork Orange, etc.)
Depends on the work, not automatically on-topic.
- If the question is specifically about an sf-nal element, even if it's only a minor part of the work, it's on-topic.
Depends on the specific question, not automatically on-topic.
- If it's set in an on-topic universe, it's on-topic.
I suppose this is the crux of the question. I'll address it later.
- If you're not sure it's SF but you think a good case can be made for it, it's on-topic.
Depends on the work, not automatically on-topic. Related to previous case.
- If there is a minor supernatural element (e.g. a fortune teller's prediction comes true, or someone sees a ghost, or a story for children involving anthropomorphic animals) but it's just a throwaway plot element that's not particularly relevant to the question, it's off-topic.
Depends on the work, not automatically on-topic. If anything, this point would suggest that setting alone isn't enough to make a work on-topic.
To address 4, I'll use a reductio ad absurdum.
Take a work that's clearly off-topic. Say, To Kill a Mockingbird. Instead of being set during the great depression (nearly post-apocalyptic in itself), imagine that the first sentence or two mention that the year is somewhere after publication, and some big apocalypse happened and threw society back into the stone age. Society has begun to recover, so there is some law, but still a lot of racism and distrust. The rest of the story could plausibly happen in such a society with little to no further modification of the story.
Should we be allowed to ask anything we want about To Kill a Mockingbird just because of a sentence or two at the very start of the book? Clearly not.