Is there a general rule that everything in one or more of those cited tags is off-topic?
No. Neither is everything with those tags in off-topic, nor is everything to which those tags could be applied on-topic.
As a general rule, I suggest that questions on such issues are on-topic, if:
- They do not primarily pertain to a specific field.
- It is a question of style (in a broad sense) and not of scientific accuraly. E.g., it is conceivable that a style guide or copy editor without special subject knowledge has an opinion on this question.
My rationale for this is that other questions are best targetted at the respective community of that field and can be best evaluated by it. While we may have some people that can answer such questions and evaluate the answers, an asker is much more likely to find an expert on this in the community belonging to that field, e.g. on the respective Stack Exchange site. Most of us couldn’t tell apart a good from a bad answer to those questions.
Conventions of Feynman diagrams or chemical naming conventions are off-topic. While some physics journals or chemistry organisations may have guidelines for these, they pertain only to a specific field.
Conventions on denoting measurement errors are on-topic. They pertain to a large number of fields and journals have style guides on this.
In case, of statistics (such as your question), it is somewhat difficult to draw the line, as it is a field of its own but employed by many other fields. But then again, many fields have to deal with chemical substances and their names. The distinction I would make here is that chemical names or statistics are still be considered to fall into the domain of chemists or statisticians, respectively. By contrast, I am not aware of a common agreement of which field is primarily responsible for measurement errors.
Yet the question was still closed (not migrated or anything like that, just closed) on the grounds that "This question does not appear to be about academics within the scope defined in the help center."
Whenever a question is closed with a custom off-topic close reason (such as yours), you see this comment. However, such a closure also comes with a comment that gives you more specific information.
Also, is there a general rule that something on Academia.SE which might conceivably also fit on another SE site should be closed here?
No. However, if a type of question is a much better fit for other sites, I would not consider it beneficial to keep them here.