This came up recently with regard to who they are testing it on:

Google have announced that they will not make their FLoC (Federated Learning of Cohorts) targeting solution available for origin testing in Europe, mooting plans to trial the technology in the region this month. The decision comes amidst concerns that the method, which forms part of the tech giant’s post-cookie Privacy Sandbox, violates GDPR and the ePrivacy Directive.

The move was announced at a meeting of the Improving Web Advertising Business Group (IWABG) at the World Wide Web Consortium yesterday (23rd March), where Google engineer Michael Kleber admitted that FLoC may be incongruent with EU privacy legislation. A lack of clarity over who will be responsible for controlling and processing data to create the cohorts, as well as the potential use of FLoC IDs and a current uncertainty over whether internet users can feasibly give explicit consent for how their information is used under the technology could put the cookie-replacement on the wrong side of Europe’s key data laws.

Kleber’s statement was later backed-up by a tweet from Chrome product manager Marshall Vale, which said that the search leader will begin a “FLoC origin trial for users in the US and select other countries, and we expect to make FLoC available for testing worldwide at a later date.” Vale was keen to emphasise that FLoC will make it to Europe eventually, writing in a follow-up tweet that Google “are 100% committed to the Privacy Sandbox in Europe.” (https://www.exchangewire.com/blog/2021/03/24/google-defer-floc-testing-in-europe-over-gdpr-fears-bytedance-acquire-moonton-for-4bn/)