Who are the apostate Popes? There were sinful Popes yes, but none have ever dogmatically defined a Heresy. Now as a Lutheran or Reformed, yes, I wouldn’t expect you to care about what the Pope says, that has been true since the 16th century. Ok, I got it.
I call a pope telling unsuspecting parishoners that they can get to heaven by paying an indulgence to be a heresy.