  Privacy preference signals are digital representations of how users want
their personal data to be processed. Such signals must be adopted by both the
sender (users) and intended recipients (data processors). Adoption represents a
coordination problem that remains unsolved despite efforts dating back to the
1990s. Browsers implemented standards like the Platform for Privacy Preferences
(P3P) and Do Not Track (DNT), but vendors profiting from personal data faced
few incentives to receive and respect the expressed wishes of data subjects. In
the wake of recent privacy laws, a coalition of AdTech firms published the
Transparency and Consent Framework (TCF), which defines an opt-in consent
signal. This paper integrates post-GDPR developments into the wider history of
privacy preference signals. Our main contribution is a high-frequency
longitudinal study describing how TCF signal gained dominance as of February
2021. We explore which factors correlate with adoption at the website level.
Both the number of third parties on a website and the presence of Google Ads
are associated with higher adoption of TCF. Further, we show that vendors acted
as early adopters of TCF 2.0 and provide two case-studies describing how
Consent Management Providers shifted existing customers to TCF 2.0. We sketch
ways forward for a pro-privacy signal.
