Correction : I said John XXIII but it was Pie XII who excommunicated him.
However, John XXIII did not cancel the excommunication order of Fr Feeney, and it can be argued that either Pie XII or John XXIII were the last valid Popes. All of their successors were definitely not Popes.
Only Pie XII or John XXIII (before his apostasy during the Vatican II council) could have cancelled the excommunication.
In this docuмent from Fr Cekada, proof that the arguments of feyneites are invalid in the light of canon law.