IS 10262 2019 specifies in scope that, it can be used for designing of ordinary, standard and high strength concrete grades.
Mix design procedures is purely an assumption base and there is table where assumed standard deviations are given up to M80 grade concrete.
Going above M50 do required knowledge of materials and experience in concrete.
Mix design which is made on paper could only be a certificate when the trials are done and desired results are achieved. in case of failure in achieving some or any property in concrete behavior, mix design need to be revised base on observations of trails and new trials need to be done after doing corrections (doing corrections is purely dependent of knowledge in it).
there is no term called highest grade of concrete to be used as per IS code, because the grade of concrete depends on the design of structure and requirement of design properties in concrete.
However minimum grade of concrete, cementitious content and maximum W/C or W/CM (Cementitious Material) ratio to be used is specified in the IS code to ensure the durability of the structure.