In 1931, the bishop of Mainz excommunicated all Nazis in his diocese. Shortly thereafter, the German bishops met together and applied the excommunication across the country to leaders and active members of the party, and those displaying Nazi symbols or actively promoting them. When the Nazi leadership appealed to the Vatican, their appeal was rejected.
Hitler was raised by a Catholic mother and an atheist father. His mother had him baptized as an infant, but even as a young teen he expressed disgust for the Church and only went to church when his mother forced him. He embraced his father's atheism and rejected the Church as an apostate before he was even 18.
So, either we respect his own religious identity, and he apostatized (rejected Christianity altogether) sometime around 1904-07, or we ignore it and consider him excommunicated with the rest in 1931. Even before that there were repeated declarations that the Nazis were incompatible with Catholic teaching and morality, and were eventually described by the Church as pantheist and pagan, in addition to being racist and immoral.
NB: there's no such thing as a posthumous excommunication. Someone's teaching can be condemned, or actions they took condemned, posthumously, but not excommunication. Hitler died having been out of the Church for forty years, his entire adult life.