Every generation has some product/ingredient that they didn’t know was dangerous at the time: tobacco, lead, asbestos, etc. What is that item for this generation?

Plastics.

Yes, all plastics. Even bpa free plastics leak estrogenic chemicals into food, and fpod is often stored in plastic containers. Even milk cartons are lined with plastic.

Teflon(nonstick coated pans and pots) arr similarly terrible

Shoes with a raised heel is bad for your knees. (Easily measurably bad. Especially for running)