Giving a complete answer here would require knowing everything along these lines that has ever been done in any US university (or knowing which, if any, specific examples of this Naulleau had in mind), 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 that was sent when a department changed 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 memo is about (I suspect that magnetic fields do not have a large place in the curriculum of the USC Suzanne Dworak-Peck School of Social Work).
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. It seems some other social work programs have also made similar terminological changes; the following blog post mentions Cal State Northridge and Smith College: Knowledge is under attack, and “fieldwork” isn’t the problem (by Philip N. Cohen, May 12, 2023, Family Inequality).