I haven't heard about a global opt-out in the browser, but I haven't really looked for that info either. I think I've heard Chrome allows extensions to "easily"

I haven't heard about a global opt-out in the browser, but I haven't really looked for that info either. I think I've heard Chrome allows extensions to "easily" hook document.interestCohort and return any value the user wants (including random values). The standard also mentions "The user agent should offer a dedicated permission setting for the user to disallow sites from being included for interest cohort calculations." but that's only for blocking specific sites from contributing to cohort computations, not for disabling globally.