On September 15, 2023 email enumeration protection was enabled by default for all new Firebase projects created after that date to boost security. When this setting is enabled, it removes the ability to link an anonymous account with an email address. Regular federated identity provider linking still works, but the upgrade path for anonymous accounts is no longer able to be upgraded to an email credential.

If you do need to upgrade anonymous accounts to an email, the only workaround until we land an SDK fix is to disable email enumeration protection on your Firebase project's underlying Cloud project. Disabling email enumeration protection will lower the security of your project.