Not true, FireFox and Safari have had them off by default for over a year now. Additionally Chrome had planned to turn them off last year but then cried "covid" which for some reason = delay... because... think of the adverts! i mean covid!
Anyway, I'm pretty sure any large websites relying on 3rd party cookies for functionality will have already experienced users blocking issues. We have and have already been forced to change, we don't do ads, but our use case is a bit esoteric in that it needed 3rd party cookies for sessions.