FLoC (Federated Learning of Cohorts) was an experimental tracking mechanism Google trialled in Chrome. Google discontinued FLoC in 2022 and replaced it with the Topics API, so the old opt-out is no longer necessary.
Historically you could opt out by sending a Permissions-Policy: interest-cohort=() response header. Because FLoC has been removed from Chrome, this header now does nothing — it is harmless if you already have it, but there is no need to add it to new sites.
The newer Topics API cannot be disabled with a single response header the way FLoC could; it is governed by the user's browser settings and the Privacy Sandbox. For general privacy and security hardening, focus on a solid Content-Security-Policy and the standard security headers instead.
