This is what the Help Center says:
How do I ask a good question?
We’d love to help you. To improve your chances of getting an answer, here are some tips:
Search, and research
Have you thoroughly searched for an answer before asking your question? Sharing your research helps everyone. Tell us what you found and why it didn’t meet your needs. This demonstrates that you’ve taken the time to try to help yourself, it saves us from reiterating obvious answers, and above all, it helps you get a more specific and relevant answer!
This does not make prior research a must, but it does show why it's highly preferred. Some sites in the network do (or did) require prior research, and explicitly place questions which lack it out of scope. This is an arbitrary question but Mathematics doesn't seem to have this particular close reason anymore:
This question is missing context or other details: Please improve the question by providing additional context, which ideally includes your thoughts on the problem and any attempts you have made to solve it. This information helps others identify where you have difficulties and helps them write answers appropriate to your experience level.
Other sites which still have a similar policy are Physics and Chemistry:
Some kinds of questions aren't allowed here:
- Do-my-homework questions: Homework questions are OK, but they must follow these guidelines. Please don't ask "do my homework for me" type questions – we only clear conceptual doubts in homework questions and will not do your work for you.
Obviously, for some sites, e.g. Puzzling, prior research doesn't make sense (for most questions). And sometimes, a short, well-written question without research does receive good answers.