Claiming ITC is illegal?

There is no such rule existing unless your operations force you.

U selfly make a inverted tax structure, while ur ITC more than opt during respective return period.u paid an amount along with gstr 3b of respective return period.It will b a single tax liability from ur part on ur gross profit arises from outward taxable supply only .It is a business rule not like as ITR rule. ur profit arises from taxable and non taxable supplier or goods.

Yes
As per Section 49(12) R/w Rule 86B :
registered persons cannot use ITC in excess of 99% of output tax liability. In simple words, more than 99% of the output tax liability cannot be discharged by using input tax credit where the taxable supply in month exceeds 50lac.

And Rule not Applicable (Exceptions) :

If the persons have paid more than Rs.1 lakh as Income Tax under Income Tax Act,
Registered person has received a refund of more then Rs.1 lakh in the preceding financial year on account of export under LUT or due to inverted tax structure.
Registered person has paid 1% tax in cash ledger out of total tax liability upto said month in Current fy.

There is no point in highlighting this unless the operations force you. it means the ruling says you generate sales. pay off a part and then utilize in case you hv no sufficient credit.