The Protected Audience API (formerly known as FLEDGE) is a Privacy Sandbox proposal to serve remarketing and custom audience use cases, designed so third parties cannot track user browsing behavior across sites. The k-anonymity enforcement feature enhances user privacy by limiting ads that can win protected audience auctions to those ads that are k-anonymous. Enforcing the k-anonymity requirement improves user privacy and limits the ability of advertisers to target specific users by requiring each ad be shown to a minimum number of users. Details for the timeline for k-anonymity enforcement is in https://developers.google.com/privacy-sandbox/relevance/protected-audience-api/k-anonymity
The initial Protected Audience proposal included k-anonymity requirements. Enforcing the k-anonymity requirements enhances user privacy by limiting ads that can win protected audience auctions to those ads that are k-anonymous. This improves user privacy by ensuring specific users are hidden in reasonably sized crowds and limits the ability of advertisers to perform microtargeting on specific users. Explainer: https://github.com/WICG/turtledove/blob/main/FLEDGE_k_anonymity_server.md