Or it's a genuine attempt to inform people that they might be troubled by what they come across.
Do you suppose that there are people searching the catalog at the National Archives who aren't aware that some things in our history may be "troubling"? Or that advising such people to gird their loins is going to soften the blow? Or perhaps we expect that upon receiving such a warning some people will leave the site rather than be exposed to anything troubling?
Standards have changed as to how much we expect people to tolerate.
I guess that's the point, isn't it? I think we're not doing anyone a favor by teaching them to be so fragile that they will not and should not be expected to tolerate anything uncomfortable.
I think that's the genesis behind the growing movement on college campuses, for instance, to protest and disrupt any visiting speaker who dares to express an opinion that is the least bit controversial. Similarly: micro-agressions, "safe spaces", and anything that I don't happen to agree with is "hate speech."
But it starts much earlier than college, e.g., don't keep score in sports, everyone gets a trophy, etc. How about preparing them to live in the real world?
Teaching kids to not be a bully is great - but it is equally important to teach them not to make a puddle when confronted with one.
One might say that we're getting far afield but I think that kids who are utterly unprepared to be uncomfortable become adults who think that they have to warn everyone else that history might be troubling.