As a person who deals with CPTSD, I can tell you that triggers are an extremely personal thing, it could be something as inocuos as a word (not even a curse or a slur, a REGULAR word) or an image. Sometimes flashbacks come back due to agression but that means agression from the REAL world, not a game or any media that's stablished as fantasy. Of course, some subjects are upseting even for people without heavy traumas.
People became obsessed with them back in the day when virtue signaling in the internet started catching on, and now we see it everywhere, it comes out as infantilizing sometimes. Like, age ratings already exist.
I personally don't believe in them, if you are playing a war game, you are OBVIOUSLY going to encounter violent situations, if you are playing a horror game, of course there will be upseting stuff, certain black comedy productions contain themes that have "triggered" me but I just don't watch them, and I don't think it should be the responsibility of the creators to look after my feelings. How are they even going to know that somewhere, PinUpCitizen is having flashbacks to her ex husband? they're not omnipresent! I don't think I have ever encounter a useful trigger warning besides the jumpscare (hate those things) and bright lights ones.
Regardless, we live in a society, so you'll have to use them wether they're useful or not.