Update: Franck linked to an answer on this from a senior SE staffer, which appears to be an official answer for all sites based on the part I've bolded:
... it is not reasonable to require a user to know for certain there is a solution to a problem before they ask it. Of course, if you believe a user is intentionally asking outlandish questions simply to evoke a reaction, you can flag or close them as such. But on any of our Q&A sites, it is entirely possible that a seemingly plausible questions simply has no solution. That in and of itself IS the answer, and saying so is helpful to anyone else who might be searching for that information
I've seen this issue on many SE sites.
If you see a question, and you have some particular knowledge or insight which means you can make a reasonable case that no-one knows, that's the answer to the question. You're the person to answer it because you're the person with the valuable knowledge or insight that proves it's unanswerable.
I've seen plenty of excellent, rightfully upvoted, accepted answers asking the lines of "We don't know, because [reasons]".
Put it as an answer, so that:
- The asker can understand why they won't find the answer they're looking for, so they know to stop looking
- If the situation changes in future, or if (heaven forbid!) something exists you weren't aware of, others can add an answer sharing this new information
By closing, you're:
- Leaving the question unresolved for the asker, possibly wasting their time by giving them the incorrect impression they might find an answer somewhere else, like Quora or whatever the lonely planet forum is called these days
- Arrogantly standing in the way of anyone, now or in future, who does actually have access to information about this you weren't aware of
For example, let's take this question and assume it's true that the relevant authorities deliberately publish no information about what is taken into account:
If we post that as an answer:
- The asker, and interested readers, can accept it and judge that the task of trying to find out is done. This is better than leaving everyone guessing.
- If in future, information is published (in an announcement, policy change, leak, legal challenge, etc etc), a new answer can be added. This is better than arbitrarily blocking new information.
What strikes me as remarkable is, in this case, people seem confident enough that it's unanswerable to stand in the way of any government official, immigration lawyer, policy expert etc etc who might think they have specific knowledge worth sharing, but no-one so far seems confident enough to post an answer stating why it's unanswerable. Odd. Choster has posted a really excellent answer summarising what little we know and why we don't know more.