I think the most common reason for closing a question is that it is not primarily about sustainability. Some of these questions can be migrated to other SE sites, but for the ones that can't be migrated we could use something like
- not (primarily) about sustainability
Furthermore I would like to add this reason
- no fact-based answers possible
for closing questions that are unanswerable because the answers would be highly speculative (e.g. this question) but at the same time I think we should be careful not to misuse this one for closing questions that ask about other people's experiences (e.g. this question).
I did some research, right now we have 27 closed questions:
- 13 off topic (of which 6 are about gardening, 2 about finances and the rest about various other topics)
- 7 not constructive (of which 2 are just too broad, 2 with no fact-based answers possible, 1 asks for opinions, 1 is a shopping recommendation and 1 question was just very vague).
- 3 migrated (2 to gardening SE, 1 to home improvement SE)
- 2 duplicate
- 1 not a real question
- 1 too localized
I just read this post on meta stackoverflow about all changes that are coming up and I've read that people closing a question can also type a reason themselves. It seems the majority of all closed questions can be captured with my first suggestion (not primarily about sustainability). My second suggestion (no fact-based answers possible) might not be necessary, at least not until a lot of invalid questions start to pop-up that cannot be closed as "too broad" or "primarily opinion-based"
EDIT2 I've just read the explanation about the default "primarily opinion based" reason.
Many good questions generate some degree of opinion based on expert experience, but answers to this question will tend to be almost entirely based on opinions, rather than facts, references, or specific expertise.
I wrongly assumed that it was for closing questions where the question itself was opinion-based, but I now see that it is for questions where the answers would be opinion-based, so there is no need for my 2nd suggestion.