The MHRD did announce that JEE Mains 2021 will be conducted on a reduced and revised syllabus owing to the decreased academic year 2020 due to Corona Pandemic. The department has invited suggestions from the parents. At this point in time, the syllabus hasn't been decided yet. So it is not still unknown what are the topics that might be included and what topics might be omitted. But yes the syllabus will be reduced. So if you are preparing for JEE Mains 2021, stick to NCERT books until the official syllabus has been officially released. But if you have already purchased JEE Advanced books, you can study from them as well because you will get in depth knowledge about each concept. Few topics might not be needed but because we do not know which topics might be reduced and which might not be, we would suggest you to not buy any books atleast until HRD comes with the official syllabus.