Giving a complete answer here would require knowing everything along these lines that has ever been done in any US university, so I can't do that. 

Here is something that I think has a decent chance of being what Naulleau was referring to, even though the details differ from the description. It isn't a matter of *field* being treated as a "trigger word" that lecturers must warn about: rather, it is a memo communicated in relation to a department changing its name from "Office of Field Education" to "Office of Practicum Education". The use of the word "field" in the context of expressions like "magnetic fields" doesn't seem to be what the policy is focused on (I suspect that magnetic fields do not have a large place in the curriculum of the USC Suzanne Dworak-Peck School of Social Work).

The memo suggests the use of "field" in the context of phrases like "field education" could be harmful or exclusionary, but does not suggest warning for the word's presence. Rather, it suggests using revised terminology ("practicum education"). This is not directly related to the topic of trigger warnings; it is more similar to arguments about replacing technical terminology like "master-slave" or "whitelist/blacklist" with alternatives that are supposed to be more inclusive.

"[A USC office removes 'field' from its curriculum, citing possible racist connotations][1]" (NPR, updated January 14, 2023, by Giulia Heyward)

> An office within the University of Southern California's School of Social Work says it is removing the term "field" from its curriculum because it may have racist connotations related to slavery.
>
>The newly renamed Office of Practicum Education, formerly known as the Office of Field Education, within the university's Suzanne-Dworak-Peck School of Social Work, is making the change in order to be more inclusive, according to a memo sent out to faculty and students this week and obtained by NPR.
>
>"This change supports anti-racist social work practice by replacing language that could be considered anti-Black or anti-immigrant in favor of inclusive language," the memo reads. "Language can be powerful, and phrases such as 'going into the field' or 'field work' may have connotations for descendants of slavery and immigrant workers that are not benign."

The article links to a [tweet by Houman David Hemmati][2] that gave the text of the letter as follows:

[![enter image description here][3]][3]

As the article notes, this memo received some public attention, which is why I think it's plausible that the idea of lecturers giving trigger warnings for the word "field" is a garbled reference to this.

Of course, there's a chance that rather than inaccurately describing this memo, Naulleau is accurately describing some other incident that I'm not aware of. As the comments have mentioned, there is no such thing as a standard list of trigger warnings that all US universities follow; this means that it would definitely be inaccurate to say without qualification that giving trigger warnings for the word "field" is something that US universities do, but it's also difficult to be 100% certain that it has never happened at all in any US university.

  [1]: https://www.npr.org/2023/01/14/1148470571/usc-office-removes-field-from-curriculum-racist
  [2]: https://twitter.com/houmanhemmati/status/1612635584539033603?ref_src=twsrc%5Etfw%7Ctwcamp%5Etweetembed%7Ctwterm%5E1612635584539033603%7Ctwgr%5Ef0643c02cb5ff7755be132e8ee5d8447074a5468%7Ctwcon%5Es1_&ref_url=https%3A%2F%2Fwww.npr.org%2F2023%2F01%2F14%2F1148470571%2Fusc-office-removes-field-from-curriculum-racist
  [3]: https://i.sstatic.net/tnYoW.jpg