Catharism seems like it completely dissippeared in the 14th century but I know other medieval Christian heresies like Lollardy survived in England until the 16th century when it merged with the Protestant Reformation, so is it possible that Catharism or some cultural elements or memories of it survived in Southern France for a few centuries from the 1300s until the 1500s and then contributed to Southern France becoming a militant stronghold of Protestantism in the 16th-17th centuries?
I've asked a few questions about Cathars on this sub, and it seems that a lot of historians question whether the Cathars even existed at all. You might want to look at this answer about Cathars from u/sunagainstgold