I do rather like the suggestion made by somebody earlier in this thread, at least for online works: 'click here if you want to get content warnings'. People who really absolutely hate spoilers can just not click.
t'was i! *does a hair flip*
as far as i can tell, it's already a pretty common thing to do, like gigi digi did for her comic "lady of the shard". i don't even read that much fanfic but i admire the content warning culture on ao3 - sometimes people will write "look at end notes for specific warnings" in the front notes of a chapter, in addition to all the tagging systems in place.
i also agree that vague warnings are often less than helpful, sometimes even misleading. the worst offender i can think of was a show that started with a very clear warning of "this episode portrays struggles with mental health" which, to me, can mean things like OCD symptoms, anxiety attacks, dissociation, and so on. turns out the episode portrayed a character committing suicide, which i did not see coming at ALL. it left me kinda dazed, like "oh so that's what they meant..."
it's okay to be specific, jeez!
(though i still think "this blog does not tag triggers" is a very fair thing to have on a personal tumblr blog, for example. if you're writing professional articles intended to be read by a wide audience, that's another thing.)
re: clarifying the genre of lovely people, i think that could help a lot, but i also think there's a disconnect here - namely that a lot of the contents that really upset people are things the author do not see as Upsetting Things. again, this could be my religious illiteracy and almost no experience with christian media, but i wouldn't have expected a christian story about standing for what you believe in to belittle my identity and throw me under the bus, y'know? it's like, trying to explain to a conservative transphobe why misgendering me is hurtful, but to them *i'm* acting hurtful for wanting them to restrict their precious freedom of speech. or asking a youtuber who posts a public meltdown to put in warnings with timestamps.
like... we are trying to figure out how a story that is actively mocking the concept of trigger warnings can be best labeled by the author who is doing the mocking. i think it's more fruitful to think of what the community can do to accomodate members, than trying to change the mind of someone who doesn't want to listen.