This is the most trending question on internet nowadays students are worried about JEE Mains 2021 because of the current condition due to COVID19 and schools and coaching centers are closed that’s why student will not going to have enough preparation time for JEE Main 2021.
So, Yes according to the sources and rumors on internet it is possible that NTA can reduce up to 30% syllabus of JEE Main 2021 and examination can be held on in August 2021 so that candidates can get enough preparation time. Also as latest announcement of Ministry of Human Resources and Development (MHRD) on CBSE class 10^{th }and 12^{th} final examination that 30% syllabus will be reduce in Examination so possibly this 30% can impact on JEE mains 2021 Syllabus.
Will JEE main 2021 Syllabus be reduced?
Yes the chances of syllabus reduction is quite high as JEE Main Syllabus is same as the syllabus of 12^{th }class and 30% syllabus is already reduced by CBSE Board. Although there is no official announcement yet from National Testing Agency (NTA) regarding reduction of syllabus but NTA can announce reduction in syllabus in Information brochure of JEE Main 2021.
List of Topic Got Reduced from CBSE class 12^{th }syllabus:
Following are the list of 30% syllabus got reduced by the CBSE board in Mathematics, Chemistry and Physics.
Mathematics 

Unit and Chapter 
Syllabus Reduced 
Unit 1: Rational and Functional 


Composite Functions, inverse of a Function. 

Graphs of Inverse Trigonometric Functions, Elementary properties of Inverse Trigonometric Function. 
Unit 2: Algebra 


Properties of Determinants, Consistency, Inconsistency 

Concept of elementary row and column Operations, Proof of uniqueness of inverse. 
Unit 3: Calculus 


Rolle’s and Lagrange’s Mean values Theorem. 

Rate of change of bodies, Use of Derivatives in Approximate. 

Definite integrals as a limit of a sum ∫ √𝑎x2 + 𝑏𝑥 + 𝑐 dx, ∫(𝑎𝑥 + 𝑏)√𝑎𝑥2 + 𝑏𝑥 + 𝑐 dx


Solutions of linear differential equation of the type, formation of differential equation whose general solution is given., ds/dy+px=q, where p and q are functions of y or constants. 

Area between any of the two above said curves. 
Unit 4: Vectors and Three Dimensional Geometry 


Scalar triple product of vectors. 

Angle between (i) two lines, (ii) two planes, (iii) a line and a plane 
Unit 5: Linear Programming 


Mathematical formulation of L.P. problems, (unbounded) 
Unit 6: Probability 


Mean and variance of random variable, Binomial probability distribution. 
Physics 


Chapters 
Topics 

Chapter 1. Electric charges and fields 
Uniformly charged thin spherical shell (field inside and outside). 

Chapter 3. Current Electricity 
Carbon resistors, colour code for carbon resistors, Combination of ResistorsSeries and Parallel. 

Chapter 4. Moving Charges and Magnetism 
Cyclotron 

Chapter 5. Magnetism and Matter 
Magnetic field intensity due to a magnetic dipole (Bar Magnet), Torque on a magnetic dipole (Bar Magnet) in a uniform magnetic field. Para, dia and ferro  magnetic substances, with examples. Electromagnets and factors affecting their strengths, permanent magnets. 

Chapter 7. Alternating Current 
Power factor, Wattless current. 

Chapter 8. Electromagnetic Waves 
Basic idea of displacement current, 

Chapter 9. Ray Optics and Optical Instruments 
Reflection of light, spherical, mirrors, Mirror Formula, Scattering of light  blue colour of sky and reddish appearance of the sun at sunrise and sunset. Resolving power of microscope and astronomical telescope, polarisation, plane polarised light, Brewster's law, uses of plane polarised light and Polaroids.


Chapter 11. Dual Nature of radiation and matter 
DavissonGermer experiment. 

Chapter 13. Nuclei 
Radioactivity, alpha, beta and gamma particles/ rays and their properties, Radioactive decay law, halflife and mean life, Binding energy per nucleon and its variation with mass number


Chapter 14. Semiconductor Electronics 
Materials, Devices and Simple Circuits Zener diode and their characteristics, Zener diode as a voltage regulator


Chemistry 

Units 
Reduced Topic 

Unit 1: Solid State 
Electrical and magnetic properties. Band theory of metals, conductors, semiconductors and insulators and n and p type semiconductors. 

Unit 2: Solutions 
Abnormal molecular mass, Van't Hoff factor 

Unit 3: Electrochemistry 
Lead accumulator, fuel cells, corrosion, law of electrolysis (elementary idea), dry cell electrolytic cells and Galvanic cells, 

Unit 4: Chemical Kinetics 
Concept of collision theory (elementary idea, no mathematical treatment), activation energy, Arrhenius equation. 

Unit 5: Surface Chemistry 
Emulsion  types of emulsions, catalysis: homogenous and heterogeneous, activity and selectivity of solid catalysts., enzyme catalysis. 

Unit 6: General Principles and Processes of Isolation of Elements 
Entire unit 

Unit 7 : pBlock Elements 
Preparation and properties of Phosphine, Sulphuric Acid: industrial process of manufacture, Oxides of Nitrogen (Structure only); Phosphorus  allotropic forms, compounds of Phosphorus: Preparation and properties of Halides and Oxo acids. 

Unit 8: d and f Block Elements 
Chemical reactivity of lanthanoids, Actinoids Electronic configuration, oxidation states and comparison with lanthanoids. Preparation and properties of KMnO_{4} and K_{2}Cr_{2}O_{7}. 

Unit 9: Coordination Compounds 
Structure and stereoisomerism, importance of coordination compounds 

Unit 10: Haloalkanes and Haloarenes 
Uses and environmental effects of dichloromethane, trichloromethane, tetrachloromethane, iodoform, freons, DDT. 

Unit 11: Alcohols, Phenols and Ethers 
Uses with special reference to methanol and ethanol. 

Unit 13: Amines 
Diazonium salts: Preparation, chemical reactions and importance in synthetic organic chemistry. 

Unit 14: Biomolecules 
Oligosaccharides (sucrose, lactose, maltose), polysaccharides (starch, cellulose, glycogen), importance of carbohydrates. Vitamins– classification and functions. Enzymes. Hormones  Elementary idea excluding structure.


Unit 15: Polymers 
Entire chapter 

Unit 16: Chemistry in Everyday life 
Entire chapter 

These are the 30% Topics form Physics, Chemistry, and Mathematics and as 12^{th} and JEE Main 2021 Syllabus are same so these topic can possibly be those topic NTA want to reduces form JEE Main 2021 Syllabus.