> “Trigger warning” was originally intended to warn about descriptions that could literally trigger a dissociative episode for people suffering from PTSD

To the extent it was, it was based on a completely misinformed idea of how triggering in PTSD works. In practice, if not in intent, it has been, from the beginning, an appropriation of the language of PTSD to serve as a vehicle for expressing personal value judgements and content preferences that have nothing to do with that. And there is research, IIRC, that it is actually counterproductive, inducing stress without helping anyone avoid PTSD triggering.