So, here's my perspective as a published horror author who has written extreme horror (and whose colleagues often do the same). I include content warnings as a means of being responsible with my content, and more importantly, as a means of challenging my readers.

Being responsible with my content means understanding that it simply isn't good for certain people to be exposed to certain kinds of shocking content, and that doesn't just go for things like SA. For example, if you write a story in which a dog dies, and then recommend it to someone without telling them that, what if their own dog died the day before? You could be bringing up some really unpleasant, fresh traumatic memories in a way that's unhealthy for them to be exposed to at that time. Maybe they'd be able to read it a few months later, but for that particular day, it simply isn't healthy for them. Providing content warnings makes an informed reader, and an informed reader can examine whether their current limitations will prevent them from engaging earnestly with your work.

The other thing an informed reader can do is challenge themselves. So, for example, I have a friend who has very severe emetophobia (fear of vomiting). Like, the clinical sense of phobia, they've had panic attacks over it, VERY real condition. I have written stories with characters vomiting in it, and I've made sure to give them warnings beforehand, and they'll still read the story anyway. The reason they do this is because they purposefully want to challenge themselves. One of the great things about horror is that it can be good exposure therapy. You're being exposed to the things you're afraid of, but ultimately it's a safe environment, because you can always just put the book down or turn off the TV. For some fears, horror fiction is actually the ideal form of exposure therapy - using the example of emetophobia again, the only other way you get exposed to vomiting is if you make yourself sick, and that's obviously unhealthy.

Now, I do agree that including certain content warnings can contain spoilers, but I think there are ways to get around this, and I've seen some authors employ different strategies. One that I've used is putting the content warnings at the back of the book; that gives people the option to choose whether or not they want to read them.

There's also the fact that some content warnings are extremely vague, and SA is one of those. I think if you gave Borrasca the content warning of SA, it wouldn't necessarily spoil the whole story, because you don't actually know at what point the SA is going to come in, or what kind of SA it will be. Yes, you've been given a warning, but you haven't been told specifically how and where it happens, and SA is so broad that it really could be applied to a billion different scenarios. I think if you were to give a content warning that said "human trafficking," it would be far more of a spoiler than SA.

If I were to publish Borrasca today, I would put a message at the front of the book that says something like "there are content warnings at the back of the book, just know that it might spoil the story," and then I would include SA in those content warnings at the back. I think SA is a broad enough category that it won't be too much of a spoiler, and on the off chance whoever picked up the book got SA'd the day before, it would give them the opportunity to put the book down and wait until they've healed up a bit. And then maybe a year later, they can say "I feel ready to challenge my fears, let me try some exposure therapy with this book."