According to the Regulation 351A of the Civil Services Regulation (CSR) the department i.e. State Government cannot initiate departmental inquiry after four years of retirement. Therefore, the departmental inquiry which is initiated after six years of retirement is violative of Regulation 351A.
Regulation 351A of CSR protects the pensioners from malicious action of the state. 351A makes it mandatory for the government to take a sanction from the Governor before initiating departmental inquiry against the pensioner and the departmental inquiry shall be in respect of an event which took place not more than four years before the institution of such proceeding. The relevant part of Regulation 351-A of Civil Service Regulations is extracted below:-
The Governor reserves to himself the right of withholding or withdrawing a pension or any part of it, whether permanently or for a specified period and the right of ordering the recovery from a pension of the whole or part of any pecuniary loss caused Government, if the pensioner is found in departmental or Judicial proceedings to have been guilty of grave misconduct, or to have caused pecuniary loss to Government by misconduct or negligence, during his service, including service rendered on reemployment after retirement :
Provided that- (a) such departmental proceedings, if not instituted while the office was on duty either before retirement or during re-employment- shall not be instituted save with the sanction of the Governor.
Such departmental proceedings shall be in respect of an event which took place not more than four years before the institution of such proceeding.
You retired in 2015 and departmental inquiry is initiated in 2021. Therefore, it is an undisputed fact that the incident in respect of which the departmental inquiry has been initiated took place much more than four years before the initiation of departmental inquiry.
In this situation the Governor cannot give sanction because the proviso of Regulation 351A specifically bars to grant sanction after four years of retirement. If the Governor has given sanction for prosecution/departmental inquiry, then such a sanction is illegal and void.
You should immediately file a writ petition in the High Court under Article 226 of the constitution of India for quashing of the departmental inquiry. The court shall not hesitate to quash this inquiry because it is initiated in violation of Regulations 351A of CSR.