This was the original idea, that some people with genuine traumatic experiences and were currently suffering from mental illness could choose to opt-out before proceeding on to read/watch something. But the concept got wildly out of hand as activists, especially younger adults, began obsessively applying it to nearly any piece of written word and anything they could frame as traumatic.
For example, if you're about to present a movie to a captive audience that involves depictions of rape, it would be good that someone who has experienced rape, especially recently, knows that it will ahead of time and has the ability to opt-out because it might trigger a traumatic episode. The circumstances where you have a captive audience and it's not clear from the context what will be depicted are actually quite rare, so its usage should be rare.
But young people, trying to signal their virtuous compassion and understanding to like-minded individuals, would put "trigger warnings" at the top of blog posts about things like "racism" or "homophobia", and all that would be discussed would be that they overheard a slur at the store.
At some point, the dominant use of trigger warnings was by people with thin skins, ready to get offended on behalf of "victims" who had suffered, at worst, nothing more than people being rude or mean to them. Pretending like these kinds of negative encounters are anything close to the mind-breaking trauma of getting raped or watching your fellow soldiers explode in front of you is disgusting, and eventually everyone caught on that the activists were trying to equivocate real trauma with "hurt feelings". Worse, they were effectively teaching young people to internalize and exaggerate negative experiences so that they could identify as someone with PTSD. That doing this made them unique and gave them extra attention from others who wanted to actively show compassion to victimized people. For lonely young people who want a cause, it was extremely attractive because it gave them identity, purpose, and community. But in reality, it was largely a perverted roleplay which coddled everyone involved and made them emotionally fragile.
The original concept of trigger warnings is solid, but should be practiced only where necessary and never attached to the phrase "trigger warning", as that nomenclature has been ceded to the activists.