EDIT: Enshittification, also known as platform decay, is a process in which two-sided online products and services decline in quality over time. Initially, vendors create high-quality offerings to attract users, then they degrade those offerings to better serve business customers, and finally degrade their services to both users and business customers to maximize short-term profits for shareholders.
I was always skeptical about cookies that didn’t serve a useful purpose to ME, and it was around 2004 when I first noticed advertising cookies. So I’d say 2003.