This makes me think though. Does this mean you have to put up tags that spoil tragic twists in the story before they actually happen?
That was discussed in some of the follow-up posts. But basically, it's entirely up to the author. If you don't mind giving people a warning ahead of time to let them know that something will happen, that's great. If you don't want to do that, then I won't hold it against you since I don't do it either. Warning people beforehand, either with a disclaimer or proper trigger warning, can help people in the same way that videos will warn people ahead of time of flashing lights. Flashing lights might trigger a seizure. A certain topic in a story might trigger somebody's PTSD. Both are catering to only the tiny, tiny minority of people who would actually be bothered by those, but it's still something that's generally seen as a good, positive thing to do.
Anyways, no. I don't think you
have to put up a warning. But if you don't, just be aware that people have a legitimate reason (imo) to complain or leave a negative rating/review if you don't warn them. Just follow whatever your personal beliefs tell you to do since there are no hard rules about this sort of stuff. Just also don't be surprised if people get upset as a result of your choice. = P
EDIT: Personally, I hate when I see content warnings before episodes of a TV show. Like you go five episodes and everything is fine, then the beginning of the sixth episode starts with a content warning advising people of extreme violence and gore. Game of Thrones did this a few times. I hated it each time. However, I understand that it helps other people, so I can't complain too much about it since I would rather somebody who can't handle that sort of content know to dip out and avoid it. Personally, when it comes to stories that might have such content, I just slap the gore and traumatizing content warnings onto them. Now, when it comes to something like completely defying somebody's expectations with what you advertised to them with the tags and synopses, that's a much more complicated situation that really just comes down to your personal belief and whether you want to deal with the probable fallout or not.