After publishing my last post, I remembered that some colleagues actually used to issue what were effectively trigger warnings when they talked about stacks, especially to audiences of number theorists. Here is a quotation from the book Degenerations of Abelian Varieties by Faltings and Chai [my emphasis]:
Because their definition [of stacks] is somewhat technical, many people seem to be afraid of and do not like them. But speaking from experience, the difficulty is purely psychological and can be overcome with time.
I believe the theory of trigger warnings presupposes that one is never completely free of the anxiety provoked by the trigger. This would not be good news for number theorists, because stacks are now popping up all over the field, from Ngô’s proof of the fundamental lemma to the Emerton-Gee paper that I covered in my graduate course last year. Fortunately, there are now many articles and a few celebrated websites that specialize in stack-anxiety therapy, and it has been years since I heard a trigger warning about stacks in a seminar talk.