Some interesting numbers, via fivethirtyeight.com:
In order to roughly gauge how common trigger warning policies were, and to find out how professors felt about the possibility of being asked to include them, the National Coalition Against Censorship partnered with the Modern Language Association and the College Art Association (professional associations for scholars of literature and art) to poll their members during the spring of 2015.1
Almost none of the more than 800 members who replied to the survey said their school required trigger warnings. Only 0.5 percent of respondents said that their institution had adopted such a policy. (About a third were unsure whether their school had a policy in place, which would suggest that the policy, if it existed, wasn’t particularly well publicized.)
If professors provided content warnings, it was most likely to be because they chose to do so. A third (34 percent) of professors said they had warned students about the content of their courses once or twice. An additional 11 percent said they had given warnings several times, and 12 percent gave them regularly.
The vast majority of professors surveyed (85 percent) said students had never asked them for trigger warnings. Thirteen percent of professors had gotten a request once or twice, and only a tiny proportion of professors polled said they received trigger warning requests several times (1.4 percent) or regularly (0.3 percent). The professors reported even fewer student movements; 93 percent of professors said they were not aware of any student-led efforts to adopt a trigger warning policy at their school.