I'm etching out a horror RPG myself. In addition to straight-up banning certain types of content, I have plans to include a modified version of this as part of a character sheet:
This way, Storytellers know which plots & NPCs your character should be kept away from. If you have a trigger for certain types of abuse, I won't be letting abuse survivor NPCs get close to your character.
I'm planning around limited visibility for PCs as well, as a way of checking OOCly if you're planning on taking a scene down a certain route. I don't want full transparency access for PCs, as that can lead to abuse.
This way, even if something is horrifying, the only way a scene can happen is if everyone in the scene is comfortable and has given their OOC enthusiastic consent to be horrified.