'email_verify_register_rest' is not a supported key.

I too can confirm this error. It appears that the issue is with the way the module stores the config settings in the user.mail and user.settings rather than creating a unique config entry for the module. Thus, to fix the error, we have to create a new method for storing the settings.

This patch requires that you uninstall and reinstall the module, so proceed with proper caution.

Before applying this patch:

1. Back up your DB
2. Backup the email message (if you have customized it)
3. Make note of your permissions settings (RESTful Web Services)

Preventing Permissions Errors:

To Prevent permissions errors, we have to either uncheck all permissions related to this module or apply patch #11 from this thread: https://www.drupal.org/project/drupal/issues/3358586 💬 RuntimeException: Adding non-existent permissions to a role is not allowed Active

Without doing either of these you will get an error when attempting to uninstall.

Uninstall the Module:

Uninstall via the UI or Drush

Clean up old settings:

Now we need to remove the old settings via Drush:
drush config:delete user.mail email_verify_register_rest
drush config:delete user.settings notify.email_verify_register_rest

Apply the Patch and Reinstall

Now we can apply the patch and reinstall the module. Reapply your permissions, re-activate the REST endpoints, and update the email message as needed.

Here's the patch

Thanks for the patch @bradhawkins. As this was a breaking change it was better to add a schema instead. Please update to the latest version and enjoy.