Sorry, but isn't this what trailers and synopses are for? These are things I check out before I decide to watch/read anything (it's often how I get to hear of them). Quite generally, these are enough for people to figure out whether something is likely to upset them. Trigger warnings are either the producers being over-cautious about lawsuits, over-egging the pudding, or a deliberate come-on, a dare if you will.

It also defeats the object of any piece of art, play, film or book that is deliberately intended by the author to upset people.