Not posting there yet, because it's a terrible environment in the comments there - so much ableism. And frankly, people who WANT readers to suffer and relive their traumas because they should "be over it already". *headdesk* I paraphrased there, but that's the gist.
The real question I have is: what is the point of including the traumatic event in a novel? Is it part of the story's concept, or just used as a plot device to create drama between characters?
I've read some of Hoover's books, and have researched the ones I haven't read. And in her case, she seems to use traumatic events as plot devices to create drama between characters.
A commenter here mentioned Jodi Picoult's books. Picoult's books differ, because the heavy topics are part of the STORY, and not just plot devices.
Here are some keywords in Picoult's book summaries (taken from Goodreads) that let readers know the possibly-triggering content WITHOUT providing a separate warning:
My Sister's Keeper: "leukemia"
The Pact: "suicide"
The Storyteller: "Holocaust"
Plain Truth: "dead infant"
I could go on, but you should get the gist of it. The problem is not about warnings, or lack thereof. And it's definitely not about "Generation Wuss", or "choosing to be offended", or "not choosing happiness". (By the way, mental illness often can't be cured, therefore the best case scenario is often management. Even with medication and therapy, sufferers may still be unable to be happy. Depression is NOT a choice. There is no gain in being marginalised.)
You know in romance novels there's something that separates the couple, before they reunite? Hoover uses traumatic events in the characters' past to break them apart.
Whereas with Picoult's books, the heavy topics ARE the story. Without them, there is NO story. But with Hoover's, you could still have a story without the heavy topics. Maybe because with Hoover's, the traumatic stuff is in the PAST, while Picoult's characters are experiencing it in the PRESENT.
In short: warnings aren't necessary, if the traumatic stuff is mentioned in the summary. And if you're just using trauma as a plot DEVICE, instead of an essential part of the STORY, perhaps you should consider your motives.
(And the best way to help people cope with trauma is NOT to just spring it upon them. There's something called "exposure therapy", I think, but that should be done with a trained professional, or with someone who actually CARES about the sufferer. And therefore NOT with those who care more about keeping fictional events secret, than causing unnecessary harm to real-life trauma survivors.)