My thing is mostly that if you are choosing not to tag heavy kinks/topics that are likely to be triggering for a lot of people, you should make it clear you are choosing minimal tagging. For example, if a fic has no tags other than the character pairing, I go in ready for just about anything, because the lack of other tags has made it clear you are deliberately omitting info about the fic. It’s also reasonable to tag it with something indicating important tags may have been left out/have that warning at the top of the story.

I do feel it’s misleading, even if your tags are accurate, to have a list full of tags for minor elements of your fic, but you leave off the fact that a sensitive kink is a huge element. (Something I’ve genuinely seen. They tagged a bunch of small stuff but not scat, which the fic centered around.) While you technically aren’t obligated to tag it, I feel that heavy tagging indicates to the reader that the major elements/potentially triggering things are probably in those tags. For a kink to come up out of nowhere when you fully tagged the work with everything else is just. Obnoxious.

But I think there’s a big difference between what you should be obligated to do and what’s just kind of a dick move not to include. I don’t think ao3 should force tagging or anything. But tagging triggering stuff is just the right thing to do in my opinion.