As one psychiatrist (I think) commented, the term “trigger” is potentially misleading, because actual psychological triggers for conditions like PTSD are not necessarily content-related.
That is, if you were brutally raped by somebody wearing a yellow shirt, for example, suddenly seeing a yellow shirt might trigger terrifying flashbacks as much as or even more than suddenly seeing images of brutal rape. Post-traumatic stress can be triggered by superficial details from the traumatic experience as well as the traumatic situation itself.
So I don’t think we should call warnings about potentially disturbing or discomforting course content “trigger warnings”. The warnings themselves are not a bad idea, but they should be presented at the start of the course in the form of a general content overview. The message needs to be conveyed that encountering material that makes you feel uncomfortable is a necessary part of learning, and students shouldn’t be encouraged to think of themselves as too fragile or broken to handle that experience.
At the same time, there shouldn’t be any stigma attached to individual acknowledgements of serious incapacitating reactions to certain types of material. Students who are doing their honest best to cope with real problems can be counseled and advised on a case-by-case basis without giving undue encouragement to opportunistic shirkers and self-absorbed “special snowflakes”.