Update from lobste.rs: Once the beta is done, there is no opt out for having your site included in the calculations.
https://github.com/WICG/floc#sites-which-interest-cohorts-will-be-calculated-on
All sites with publicly routable IP addresses that the user visits when not in incognito mode will be included in the POC cohort calculation.
This “opt out” will cease functioning as soon as the trial phase is complete, and currently it only functions if you’re serving advertisements.