Giving a complete answer here would require knowing everything that has every been done by any US university related to this topic, 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 what you describe. 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 this context could be harmful or exclusionary, but does not suggest warning for its presence. Rather, it suggests using revised terminology. Rather than being related to the topic of trigger warnings, this is more similar to arguments about replacing terminology like "master-slave" or "whitelist-blacklist".
"A USC office removes 'field' from its curriculum, citing possible racist connotations" (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."
As the article notes, this memo received some publicity, which is why I think the idea of lecturers giving trigger warnings for the word "field" could be a garbled reference to this.
The memo from the USC office, which quickly gained attention on social media, is being met with both support and derision.
The article links to a tweet by Houman David Hemmati that gave the text of the letter as follows:
