But even if you don't believe that FE has anything to do with Catholicism, there are still other subjects that are discussed on the forum that don't have to do with Catholicism. Are those subjects to be censored as well?
You are not being censored. You can still write what you want in the proper subforum. You are being made easier to avoid by the people who wish to do so.
I have been involved in online discussions for over 25 years and seen a lot of moderators/ forum owners. I don't think I have ever seen one who would allow a place for flat earth discussions. The typical reaction is that it's wrong, it's stupid, and it makes one's forum look bad.
This forum is different because Matthew's vision for it includes a wish to allow "fringe" topics even when he himself disagrees with them. People don't call Cathinfo "the Wild West of trad forums" just because the owner is from Texas. I don't know of another trad forum that would allow you to post on flat earth as you have.
Instead of showing gratitude the flatearthers are complaining about how they are mistreated and censored, maligning Matthews motives, and implying he made this forum upgrade because he was stupid enough to get tricked into it.