While I think it can be a good idea to push the envelope and challenge people's sensibilities, when specifically talking about writing out taboo things on this site I think it has to take a backseat to the forum rules. Introducing traumatic events or using things like rape as a tool to demonstrate evil can very easily be considered a violation of the forum rules depending on the context. And while you can pretty creatively dance around the exact subject to be tactful, obviously the more detail you go into the more visceral and offensive it becomes. So trying to go for something as shocking as possible might be something you can do in your own personal writing, its perhaps a good idea to leave certain taboo things alone here unless you're willing to be so tactful about it that you lose out on the shock value anyways.

Like its been said already, just touching upon taboo concepts purely for shock value is different from using them to affect the narrative. What makes the distinction is whether or not there are any real consequences or results. For example lets say you wanted to demonstrate how evil a particular character is so you write in a rape scene (to some degree of detail). If this never gets touched upon again all you did was create shock value. There's no actual consequences, nobody brings it up again, it becomes a scene that could just as easily be removed and the narrative wouldn't change at all. The character was already established evil, adding more to it means nothing if what you add literally changes nothing other than shocking your readers. Nobody's gonna think "oh well he's demonstrated capability/willingness to rape so he's even more evil than I thought", there's gonna think something more like "why was that in there? that's just gross/offensive/wrong/etc" because it makes no sense within its own context. This holds true for other taboo concepts beyond rape or other sexual violence, though obviously to varying degrees of backlash. Adding a scene to demonstrate how racist somebody is but never following up with that does the same thing: provides no consequences and no resolution, it could be removed without changing the narrative at all.

One "taboo" thing I see get tackled incredibly often is mental illnesses as a broad concept. How frequent it comes up in roleplays leads me to think it isn't really taboo at all, at least not for writing, but I still want to touch on that subject because I think that its very regularly done quite poorly. The thing is, "mental illness" isn't itself a mental illness. Having a character be "mentally ill" in a nondescript way makes no sense and is often written out similarly poorly. The idea of adding it into a narrative to make characters more interesting and less predictable is a fine one but often poorly executed because of the vague nature its usually touched upon with. Character A being "mentally ill" but demonstrating cartoonish levels of insanity is not an interesting nor tactful way to add the concept to the narrative. It basically turns Character A into a joke, and I'm sure all of us have seen this type of character in our roleplays before. I'm tackling specifically mental illness here but my upcoming concept applies to many taboo subjects and its that not knowing what you're writing about absolutely shows. And I think if you're gonna try to touch upon taboo subject matter just for the sake of adding one more element to your story without trying to put in any real effort to even knowing what you're talking about then that is no different from doing it purely for shock value.