A levels and GCSEs next year in this house, but I doubt that either DC would dare mention it if someone had told them that there was no need to revise x, because it definitely won't come up - they and their sibling must have heard me say a hundred times that if it's on the syllabus, it could be on the exam paper. Make a grown up decision not to bother with a topic, fine, but it's no-one's fault but your own if you find yourself short of questions you can answer.
Of course, they might just decide to follow 'someone's' advice and not tell us...