Well, that's kinda what I meant by saying it's futile. Yes, you could add/imply it, but I'm pretty sure it wouldn't satisfy 95% of those, who'd play it for the incest. I mean, it's usually about the taboo, about the secrecy, etc. and if the story doesn't at least partly focus on that, it's just another attribute for some character.
It's a bit like let's say BDSM content or such: Yes, it's easy to add the tag, but if you actually produce content, that's worth having that tag, is another question.
To be fair, Patreon didn't care until their payment processors made them care. It's indeed mainly about for example paypal and stripe, who don't want to support platforms, who allow that kind of content. Hence for example, why subscribestar still doesn't offer paypal payments.
Else one can indeed say, that I'm not aware, that Patreon actually ever banned somebody because of stepcest. They just wouldn't allow it as a solution to get your already banned account back.
It seems like Patreon generally doesn't really care about incest, unless you advertise it on your patreon page. Yet, there's of course always a risk remaining.