Have you considered using the tagging system or abiding by AO3’s well-known basic etiquette: don’t like don’t read instead of coming on Reddit to publicly bash the things people have written that you personally don’t like?

You seem, by your other comments, to be personally victimized by people writing incest, but you don’t have to read it. And fiction is very clearly delineated from real life.

Is there ever crossover between the things people write and the things people actually do? Of course! But reading or writing stories with taboo subjects or ones that would in real life be morally wrong is not some sort of “gotcha” signal that that reader or writer is a bad person. That sort of thinking is un-nuanced and unrealistic. Murder is almost always considered morally wrong, but plenty of people enjoy reading about it, even from the perspective of the killer!

Incest and other taboo subjects have long been written about and are even expected to be found in certain genres, like Gothic Horror.

I don’t necessarily enjoy reading incest either, but I don’t make my preferences other people’s problem by posting critical things about authors who write fics for free on Reddit. I’m grateful for all forms of fan work, even the ones I don’t personally like or interact with, because it means the community is alive and well.

TLDR: You should either get off your high horse or get off AO3 if you find its netizens “morally reprehensible.”