As far as I know, there is no research on the therapeutic value of "trigger warnings". According to a 2014 news article published by the American Psychological Association, "[t]here's no research yet on trigger warnings in the psychological literature, so psychologists don't know what effect they might have".
Which isn't too surprising considering both how recent the idea is and how difficult it would be to operationalize it.
On the matter, Veraldi and Veraldi (2015) talked at 31st Annual Symposium in Forensic Psychology regarding trigger warnings and in their text they affirm that "research suggests that identifying potential trauma triggers is far more difficult than looking for explicit depictions of violence or rape. Even seemingly neutral images associated with past trauma may evoke painful or violent responses".
Heterogeneity is a recurrent characteristic of triggers, along with the fact that PTSD can have different symptoms.
"For people with PTSD, it is very common for their memories to be triggered by sights, sounds, smells or even feelings that they experience." (Mental Health America)
Examples include: (WebMD)
"Re-experiencing symptoms may cause problems in a person’s everyday routine. They can start from the person’s own thoughts and feelings. Words, objects, or situations that are reminders of the event can also trigger re-experiencing." (National Institute of Mental Health)
- A certain smell, sound, or sight that reminds you of the event you experienced
- The anniversary date of the event
- Seeing or reading a news report about it
- Seeing a person related to the event
- Bad dreams or nightmare
And then one should also consider that avoidance is a short-term solution that in the long-term can be damaging, which could bring the ethical question of whether a study could facilitate for a group of people suffering PTSD avoidance via warnings, instead of aiding them learn to manage their symptoms and face their triggers, in order to test the effects of trigger warnings.