They are mechanistic rules that only tangentially have to do with academia.
While I agree that they somewhat differ from the majority of questions here, citation style is something that primarily concerns academics. Hence, I do not see why it should be off-topic. If you specifically do not want to see such questions, you can simply ignore the tag.
Whether you choose to have sentence capitalization or word capitalization, use a comma instead of a colon, or have the citation inside the quote or outside -- the answer is the same: look at your style guide.
Taking a brief look at the questions tagged citation-style, I found none that looked as if it could be answered with “look at your style guide”. Rather the questions are, e.g., about handling rare cases not covered by some style guide, the rationale behind certain styles, or the exegesis of a popular style guide.
If a question could only be answered with “look at your style guide”, I would indeed consider it a case for closing as depending on individual factors.