I think it's somewhat unreasonable to believe that, in the sense that Apple is pretty explicitly using their clout to make it much harder for companies to do this.
When a company blocks Mailinator, they're giving up a pretty small number of users -- those who know about Mailinator, use it, and refuse to sign up for a service that blocks it. Since Sign In with Apple is required for any iOS app that supports 3rd-party login, any company (with an app, which is mostly what this is all about anyway) that decides to block this new type of authentication will be forced to either give up all 3rd-party login options (costing them existing users who sign in this way and increasing the cost to acquire new ones), or give up their iOS app -- in either case, this probably means giving up a large number of users.
When a company blocks Mailinator, they're giving up a pretty small number of users -- those who know about Mailinator, use it, and refuse to sign up for a service that blocks it. Since Sign In with Apple is required for any iOS app that supports 3rd-party login, any company (with an app, which is mostly what this is all about anyway) that decides to block this new type of authentication will be forced to either give up all 3rd-party login options (costing them existing users who sign in this way and increasing the cost to acquire new ones), or give up their iOS app -- in either case, this probably means giving up a large number of users.