All the exam boards have published advanced notice of the major topics that will be covered in this years exams. These are meant to provide some help in preparing for the exams. I think the help that these give at A-level Chemistry is questionable. If you choose not to revise certain topics that are not on the major topics list, you could end up with a lower grade.
The key part of the guidance used is this phrase: “Topics not included on the list may still appear in multiple-choice items, questions with a low tariff, or via synopticity.” So any topic could appear on a multiple choice or as a minor part of a question on another topic. The difference in number of marks between a major topic and a low tariff topic is not specified- so potentially there could be anything up to 5 or 6 marks on a topic not listed. For example, with AQA, NMR does not feature on the major topics listed, but it is easy to add an NMR question worth a couple of marks onto almost any organic question- so you can be sure NMR will appear somewhere. Even if the major topics make up 80% of the marks, you cannot afford to throw away 20% of the marks if you want a high grade.
In addition, Chemistry is a subject that makes most sense when you have revised everything and make the connections between topics. If you take short cuts in your revision, you will be less able to answer the synoptic questions that range across topics.
So in conclusion, if you want a high grade and have a university place dependent on it, then my advice is revise everything to the same degree.