St. Joan wasn't excommunicated; she received holy Communion before her execution. In any case, her trial was manifestly unjust and illegal (her appeal to the Pope was ignored), and hence a sentence of excommunication would have been invalid. Moreover, unjust excommunications, while they separate a man from the communion of the Church, do not result in damnation, for "the person [unjustly] excommunicated should humbly submit (which will be credited to him as a merit) [. . .] if he submit humbly, the merit of his humility will compensate him for the harm of excommunication" (St. Thomas, Sup., q. 21 a. 4).
As was Archbishop Lefebvre's sentence.
Heck, New Rome didn't even give him a trial.
Wrong, again
After the reading of the sentence of excommunication came a long pause, for a condemned person was not denied time to address the people if wishing to do so. For half an hour or more Joan spoke, protesting her faith and trust in God, asking for the prayers of the people as well as for the intercession of the saints, and her words, "pitiful, devout and Catholic", were so moving that those who could hear her, even the Cardinal of England and many Englishmen, were seen to weep.
The soldiers grew impatient. Two sergeants came and forced her down from the platform where she stood and led her to the Bailiff who represented the English authorities. So far she had been excommunicated but not sentenced to death: yet no judgment was read in the name of the king, no sentence was pronounced, and the Bailiff, merely waving his hand, to signify these legal formalities were not worth troubling about, said: ""—that is: "Take her away. Take her away"—and she was straightway taken to the stake and handed to the executioner. She asked for a cross and a soldier hastily made one with two pieces of wood tied together—she kissed it and put it in her bosom. Then her arms were pinioned behind her back and she was chained to the stake. At her request, Isambart, who, as well as Ladvenu, was attending her, sent for the cross of a near-by church and held it before her right to the end of her long agony. "To the end of her life", affirms Martin Ladvenu, "she maintained and asserted that her Voices came from God and that what she had done had been done by God's command. She did not believe that her Voices had deceived her, and in giving up the ghost, bending her head she uttered the name of Jesus in a voice that could be heard all over the market-place by all present, as a sign that she was fervent in the faith of God." Her heart was unconsumed. By order of Cardinal Beaufort, the ashes and all that remained of St Joan were put into a sack and thrown into the Seine "that the world might have no relic of her of whom the world was not worthy".[5]