It would break things like medium.com's constant prompt to login with this list of google accounts, but it wouldn't break SSO - SSO itself is typically handled

It would break things like medium.com's constant prompt to login with this list of google accounts, but it wouldn't break SSO - SSO itself is typically handled by a series of redirections, and at the time you're redirected _to_ the SSO server, you're giving your cookies to _them_, then you're redirected back with a signed response and get cookies for the _destination_ domain - at no time during the exchange does the SSO server need to directly talk to the relying party. Implementations may of course vary.