Content warnings, to my mind, are very general. They flag, usually, fairly generalized things - material inappropriate for children, especially, but then fairly general information about violence or sexual material. Film ratings being the most obvious examples. It's really so people can pick entertainment that they want or don't want, more than anything.
As far as educational institutions, or most public lectures on science or historical topics, I think this is really something for individuals to manage. If someone is suffering some kind of psychological problem that means they can't read about or hear about things like elements of the law, it may be that the law isn't really an appropriate career. Similarly history, or science.
The level of content where you are talking about really likely bothering many people is where a teacher might think, maybe I don't really need to show this in this setting. There will be exceptions but they should be rare.
Aside from the setting people up to fail element, avoiding upsetting, anxiety producing, or otherwise traumatizing content, for individuals, usually isn't a viable long term way to manage that kind of reaction. Avoidance typically makes things worse.
Where people have active, unmanageable reactions, they are going to have to be proactive about dealing with that, not rely on what someone might think to warn them about.