One tricky thing here is that existing literature is really examining the potential effects of trigger warnings in and of themselves, devoid of context or non-immediate decision making. Does seeing a literal trigger warning make someone feel less anxious? Almost certainly not, why on earth would it?
In studies that find no or slight negative effect, the outcomes are immediate measures. How do you feel right now? If it assesses decision making, it’s whether you do or do not immediately consume the content.
But for trauma survivors the potential to be triggered is always in flux, always dependent on everything else going on in your life, often set off by things that seem unrelated or irrational. Trigger warnings give someone a choice in that exact moment for what to do based on what they believe they can* manage. Yes, it may promote avoidance, but avoidance can increase feelings of agency that allow for reduced avoidance behavior in the future.
As an example from the great college campus syllabus trigger warning kerfuffle: I assign chapters from Durkheim’s Suicide in some seminars, as well as complementary readings with less obvious titles. My students get a warning about this ahead of time, but they don’t get to just skip that part of the class. Some things students have done: scheduled extra therapy sessions during those weeks, read in small groups in the library instead of isolated in dorm rooms, missed a class meeting and made up for it with office hours and a short additional assignment (so they didn’t out themselves to their peers with a panic attack in class). It’s about agency and self-assessment.
A screen with a suicide hotline number isn’t going to magically make someone ok with seeing suicide represented, but it offers an action the person can take to regain agency.
*Or just want to manage. Sometimes you’re just living your life and not super in the mood for exposure therapy, and if you can get your brain somewhere else for a while that’s a very good thing.