Understanding Topics API in the Wild: Dubious Usage and Stale Adoption

Among several proposals for a privacy-preserving replacement for third-party cookies, Google's new Topics API is widely discussed as a possible solution to balancing privacy and utility for online targeted advertising. Despite being promoted as a key solution in a post-cookie world, the new paradigm still faces skepticism from researchers and regulators. This paper provides a first complete independent study of current adoption of the Topics API technology in the wild, to observe whether popular advertising platforms are already deploying it and how. We study the practices they adopt and their interplay with privacy policies and consent acquisition mechanisms. For this, we deploy a crawler to record the usage of Topics API across tens of thousands of popular websites worldwide, enriching our results in a twofold dimension: first, we run a seven-months-long campaign to understand how the adoption of Topics API is evolving over time---showing that typical problems of early deployments are still to be solved. Second, we observe how the use of the Topics API changes when observed from different regions, each adopting different privacy regulations. We find that users are likely to encounter the Topics API while surfing the Web, regardless of the geographic position. Our results show that this technology has not yet replaced third-party cookie technology for targeted advertising.