As of May 2023, there is an extra step in binding a Permission Set to an External Named Credential, as the Permission Sets list vanished in favour of an intermediary called Principals. To accommodate this, all you need to do is create a Principal record first (choose any Parameter Name you like), add your Authentication Params there, then after saving, navigate to the Permission Set and bind via the 'External Credential Principal Access'.
As of May 2023, there is an extra step in binding a Permission Set to an External Named Credential, as the Permission Sets list vanished in favour of an intermediary called Principals. To accommodate this, all you need to do is create a Principal record first (choose any Parameter Name you like), add your Authentication Params there, then after saving, navigate to the Permission Set and bind via the 'External Credential Principal Access'.