Input igst wrongly taken instead of cgst/sgst in march 2021

This is a litigative issue since claiming period of ITC has passed. The law also doesn't specifically allows this treatment as it allows in the case were wrong tax was paid as in section 77.

Therefore you will have to reverse the wrong ITC but to claim the ITC is a litigative matter

You have to reverse the ITC wrongly claimed under wrong head. The correct ITC pertaining to CGST and SGST head cannot be claimed after the time limit for FY 20-21 has expired.

You should reverse the wrongly claimed ITC while filing the GSTR 9. 

Further, you can claim the CGST/SGST ITC but that might be litigated by the GTS officer in assessments. 

Bonafied inadvertent human error. Revenue neutral. No loss to revenue. No extra benefit derived by tax payer. Only procedural lapse in making disclosure under wrong head in Gst return 3B. Few court cases also said in such case interest of justice be served. 

Any case law supporting the same