It's easy to point to any one example and say "Well that could still work." but the point I was trying to drive at is that this changes the dynamic of the lecture. Surprise can be used as a catalyst for introspection or introducing a strategic amount of discomfort to illustrate a point, etc. the point I was trying to make is simply that it has utility. When you are /required/ to have a trigger warning, as an instructor, you are being obligated to compromise the effect you were trying to persue and presumably you were trying to persue it because you thought it would help the class with some form of comprehension whether that be intellectual or emotional.
Even if you're not outright required to issue trigger warnings (lets say because a middle ground policy is implemented that encourages instructors to do so but leaves it to their discretion) those instructors are still required to adapt to triggered students or else they wouldn't have been encouraged/required to do so in the first place. But because there's no reliable way to know what specific content will trigger a student in a given class the system is actually incentivizing censorship because now the only way to ensure you minimize trigger events is to assume that every student is triggerable and so optimal content becomes content which is as unchallenging, emotionally speaking, as possible.
Allergies wouldn't be a terrible analogy. Lots of schools, in America I can't speak for Europe and elsehwere, have regulations about common allergies like peanuts. These regulations are targeted towards common allergies because otherwise the task of preventing allergic reactions becomes an all encompassing battle to minimize liability. That's not to say students with rare allergies shouldn't be accommodated at all, but at some point the impact on the other students has to be addressed.