Isolated Witness – Do I need a BIP to remove the SegWit discount for inscriptions?

Removal of the witness discount would result in a consensus change, especially a soft fork. Changes to the consensus rules have always been standardized in the BIP since the BIP process was created, and we expect this to continue to be the case for the foreseeable future, but this is not the rule. Anyone is free to persuade anyone to implement consensus changes without a BIP.

However, it is unlikely to have much success in persuading people to undo one of the major improvements introduced in SegWit. The basis for witness discounts can be found in the following questions and answers: