Do you know if TFSI is harmful to human health? Or was it merely investigated as a tracer?

A little of both. Anion bis(trifluoromethane)sulfonimide (TFSI) at higher concentrations it can be hazardous (chemical burns). Was used as a tracer as it is another chemical that 3M made/used/disposed at the location in question. As PFAS is ubiquitous, this provides a way to ID the pollution plume.