Might be overthinking this. The instance rules already don’t allow most IRL content and anything that looks underage, so that more controversial stuff is not even an issue here and it might be a little redundant to make another rule about it. Probably the most edgy thing that’s left to worry about are drawings of zoo/beast stuff, which you could probably exclude just on the basis that it’s off-topic or no longer considered “furry”.
Edit: If things ever get too confusing, I always bring it back to the fantasy vs. reality standard. It’s one of the few principles I’ve found that holds up to scrutiny and helps moderators maintain their sanity. Otherwise it’s way too easy to justify or rationalize any number of reasons why a particular kink is right or wrong, and it usually devolves into how the individual feels in the moment.
Yeah, I vehemently disagree with some of the opinions expressed in that thread once it started going in a different direction. But I still think there is some value in sparking a good faith debate on such difficult subjects. I hope that this place can still respectfully discuss unpopular opinions like that in the future without being too heavy handed moderation-wise.