I've seen some proposals like "Allow custom close reasons that are off-topic" floating around on meta.
My question is, until those proposals are implemented, can I use custom close reasons for things other than "off topic"?
It seems fair to use them for things like mentioned in that proposal ("cannot reproduce") - that's a reason to close that doesn't fit into any existing close reasons. If there's an automatic downvote that's unfortunate, but still, seems it's the best the system lets us do.
However, they shouldn't be used to replace already-existing close reasons. So, if you're tempted to write a custom close reason, just look at the non-custom ones first. Only use a custom reason if none of those fit.
If it's a mix of standard close reasons (say, broad and opinion-based), there's probably still a dominant one; use that one and explain in a comment. That way we can take advantage of the prominent UI treatment and standard language for the closure. In particular, be careful not to use a custom close reason simply to explain why a question is, say, too broad and opinion-based. That can be done with a standard close vote and an explanatory comment.