4. PTSD is a disability; as with all disabilities, students and faculty deserve to have effective resources provided by independent campus offices that handle documentation, certification, and accommodation plans rather than by faculty proceeding on an ad hoc basis.
Colleges and universities should and do have systems for dealing with disability accommodations, students with disabilities including PTSD should make use of them, those offices should consider how to make appropriate accommodations for those disabilities, and professors should (and presumably will) make those accommodations. That's very different from asking professors to anticipate a range of content which might upset some students and regularly warn them about it. In conversations around this subject there seems to be this notion that we all know what might be disturbing, but especially in the context of actual PTSD, in which triggers can be anything, that's completely untrue.
And if mandatory trigger warnings on everything are the policy, it's the equivalent of stamping "this product may contain nuts" on every food item. Not actually helpful to the people who really need the information.