While I haven't read the entire thread, I figured I'd respond to the first post, since I have pretty particular ideas about this sort of thing.
I think that ultimately, this sort of thing boils down to communication and trust. If you know that your content is generally considered unacceptable or triggering for a large portion of people, then warning them ahead of time before they even enter the story, that a plot might contain certain hard themes, is only respectful. This is a collaborative hobby that's supposed to be fun for people, so I think that it's only fair to have other people in mind when doing this sort of thing.
That said, you can't be prepared for everything, nor can you really add a laundry list of warnings, like, "Warning, there will be rabbits", just in case someone is afraid of rabbits. You warn for the most commonly accepted triggering things. If someone has a problem, they should politely bring it up, though this can be troubling if you're in a plot run by someone you don't trust or feel comfortable with, which is why I generally suggest to have a healthy caution about knowing at least something about the person running your plots, and if they're cool to communicate with.
That said, if you are the runner and someone brings something up to you, I think that being polite and having a discussion about how they can accommodated is important. If you can't compromise for that particular scene, do what I've done in the past, which is promise that you'll have a place for them in a future scene. Sometimes it's all you can do, and hope that the player understands.
At the end of the day, communication is what matters. If there's no mutual communication, trust, and respect, then it's very difficult for everyone to have fun. And that is the key here, the point is for people to have fun, because this is a hobby where we all just wanna do our shit and feel awesome, and possibly escape the shit we deal with irl.
I recall an extreme case on Shang of someone saying they were triggered by semen. I have no idea what you do with that on Shang, to be honest, but there are limits. Like, you don't play Megaman MUSH if you find robots triggering, or Fallcoast if Mages and Vampires trigger your PTSD. People can only accommodate within reason, but obviously these are, as I mentioned, very extreme cases.
I also believe that certain MUs and communities find certain things more/less offensive or unacceptable than others. I think this is perfectly fine, not every community is the same. But I think that as human beings, you can't really argue, "Alright but I should have the right to just do my rape plots without warning anyone", that's kind of a toxic 2005 dick move in my opinion. If you wouldn't walk into a crowded bar and yell, "HEY, EVERYONE I'M GONNA TELL YOU THIS STORY I WROTE ABOUT RAPE", you shouldn't do it in a MU.
Incidentally, I know exactly how a bar responds when someone starts singing a song at an open mic about bears raping people (Spoilers: EVERYONE GETS SUPER UNCOMFORTABLE!)
tl;dr Basic social skills go a long way.