Does all horror need a content warning?

I typically say "no horror" when I put my name on the beta list, but this time I had to write horror, so... I wrote something that wasn't overtly violent and focused instead on psychological horror, not graphic.

I did decline to beta a story that triggered some of my PTSD, but there's no way the writer would have known about it as it was personal to me, not a common trigger. And that author wasn't writing horror, so it wasn't on them to post any warnings, IMO. It was on me to back away and let it go.