The question, Properties of MLE (minimal sufficient and complete) on a restricted parameter space was, per the comment, already posted on the math site. Moreover, people have been voting to close it (review queue page). I think it belongs here (seems to be statistics, pretty clearly). But then it should be closed on the math site here. I'm not sure what to do here.
If the question's on-topic here I most usually comment, adding a link to the cross-post & asking the OP to decide which site they want the question on. I don't close it, partly because we don't have a relevant close reason,† partly because that makes for more work later if the OP removes the cross-post & wants it re-opening here, but mostly because it seems to me a heavy-handed response to a minor transgression. Once the questions are linked the worst that can happen is that the OP neglects to delete one of them, & other users make the informed choice to provide similar (or different) answers on both sites. The more important thing is to explain what the rule is so that the OP can abide by it in future.
Sometimes I just delete the question; if it's unanswered, has been here a while, & if the cross-post already has an answer. Rarely I suggest splitting it or tailoring it into two questions, each more suited for one or the other site.
There are so many SE sites now, & so much overlap, that it doesn't seem worthwhile to spend time researching whether the question's on-topic both here & on the site of the cross-post— though if I do happen to know enough to have an opinion on the most appropriate site I might share it. (I believe Mathematical Statistics is on-topic at the Maths site, for what it's worth.)
Deliberately & persistently flouting the rules is another matter, of course.
† There's an "Other" close reason, as @gung suggests using, but the boilerplate reads
This question does not appear to be about statistics within the scope defined in the help center. If this question can be reworded to fit the rules in the help center, please edit the question.
, which comes across a bit oddly. And what do you do if the question already has an answer?
It's a common issue with the current way Stack Exchange is designed, and it's even getting worse with the creation of the AI and DS websites, which largely overlap with CV. I think the best solution is Build and strengthen the Stack Exchange community with "crossover questions" between sites.
When I come across such a question, I close it. I leave a comment along with closing, asking the OP to decide which site they prefer and delete the other version. They could then have this version reopened, if they preferred. I don't usually downvote. As @Scortchi notes, these OPs are usually new and simply unfamiliar with the SE system.
We can close questions as duplicate and refer to an existing question on stats.stackexchange, we should be able to do the same when there is a similar (cross-posted or not) question already existing on another stackexchange website (currently it is only possible to deal with this in the comments and you see this for instance a lot between math-overflow and math.stackexchange).
The current way how the question has been closed is confusing. I would say that it is not off-topic, but the off-topic category has just been (ab)used in order to close it.
Sure, the way that it is handled is ok, and the clarification in the comments explains sufficiently why it has been closed. Also Gung explains well how this category can be used more ambiguously.
But for an average user that encounters this situation again and wonders how to deal with closing a cross-posted question, it will be unlikely that he/she is aware of this meta-question or is gonna find it quickly.
If cross-posting is a good reason to close a question then this solution should be more clearly available in the options to close a question (instead of currently being ambiguously contained in the off-topic section, while not strictly being off-topic).
If it is on-topic on both sites but the sites do not liaise and it gets closed on both sites it is difficult to feel too much sympathy for the OP since we should not be encouraging cross-posting which potentially wastes people's time. In this case the OP has a clear message in the comments about why people have closed it so knows what to do if they want an answer.