Caution: The following requires that you have “Keychain” and “iCloud Mail” activated for your Apple ID, and are running iOS 15+, iPadOS 15+, and macOS Monterey+ on the applicable devices.
For macOS / iOS / iPadOS users you can use “Hide My Email” (aka “Sign in with Apple”) to have a randomly generated e-mail address used for sign-ups. Then you can have Keychain generate a random, strong, passcode for you to use on that particular website.
Note: If you see just an “e-mail” field on the sign-up page, and when you click in it you see a drop-down menu of your current Apple e-mail addresses, just scroll to the bottom of that list to get to “Hide My E-mail”.
After you’ve completed a sign-up on a website (not just federated, but any website) Apple will e-mail you the new e-mail address you generated and for which website is was used on. Store that in a safe place to easily keep track of what’s being used where. (You can also find them here: System > Apple ID > Hide My E-mail > Options…)
All of the above, the randomly generated e-mail addresses and passcodes for the websites, are automatically synced through iCloud to all of your devices signed into the account used to create them.
Note: The “Sign in with Apple” button is a bit weird. When you’ve used it on a website, then later go back and are asked to sign-in to your account, you’ll see the same “Sign in with Apple” button you saw originally. There’s no visual indicator on the page that you’ve used this feature before. It’s not until you select the button that the next screen will show you the e-mail address that was generated.