The title pretty much explains the question but here it is
Is it off-topic to ask for advice on what courses/subject areas to study in preparation for PhD study in a certain area of mathematics?
The question is really about the closure reason
Questions seeking personal advice for choosing a course, academic program, career path, etc. Such questions should be directed to those employed by the institution in question, or other qualified individuals who know your specific circumstances.
Is it "seeking personal advice" to ask what topics are worth studying when preparing to undertake PhD study in a specific area, or is it beneficial to others who may be seeking similar advice?