Is a sister who swore, saying, "I will die an unbeliever if I do such-and-such," and then did not do it for the general good, required to perform an act of expiation for her sin after many years have passed?

It is not permissible to swear by anyone other than Allah, and it is considered one of the major sins, as the Prophet (peace be upon him) said: "Whoever has to swear, let him swear by Allah or keep silent," and "Whoever swears by other than Allah has committed Shirk (polytheism)." As for swearing by disbelief (Kufr) or disavowing Islam, it is even more forbidden and carries a greater punishment, as the Prophet (peace be upon him) said: "Whoever swears by a religion other than Islam, then he is as he said." This type of oath has no expiation according to some scholars due to its gravity, while others believe that it does entail an expiation for an oath. It is advised to stay away from such oaths, and to offer expiation for them as a precaution, and to increase in repentance, seeking forgiveness, and performing good deeds.