Earlier quoted context omitted.
This warning is implied for all information, all the time.
Originally this comment irritated me a little, but to be quite honest I think it is reasonable and desirable to retrain everyone to think this way. If some content is a big problem for you, then you can and should definitely stop consuming it - warning stickers aren't going to help you do that and will never cover the nuance of sensibility you have as a real human being anyway.
The problem people have with my comment is that it undermines the implicit assumption that 'authorities' are inherently trustworthy. Sense of trust, sense of authority, those are very powerful emotional motivators. Challenging those ideas is cause to challenge one's self, and that's uncomfortable for many people.