What is the ruling on Mu'awiyah's cursing of Ali (may Allah honor his countenance), and the cursing of him by the Umayyad caliphs, until Umar ibn Abd al-Aziz abolished that?

It has not been established that Mu'awiyah (may Allah be pleased with him) cursed Ali ibn Abi Talib (may Allah be pleased with him); rather, what is established is the occurrence of fighting between them at Siffin. The creed of the People of Sunnah is to refrain from delving into the tribulations of the Companions and to believe that these events occurred due to ijtihad (independent reasoning), and that the one who is correct receives two rewards, while the one who errs receives one reward. Their motto is the saying of Umar ibn Abd al-Aziz: "That was a tribulation from which Allah protected our swords, so let us protect our tongues from it." It has been reported that Mu'awiyah prayed for mercy upon Ali. As for the hatred of some Umayyad caliphs towards Ali and his household, this has been mentioned by scholars, and whoever among them did so was not known for knowledge and piety.