preferEphemeral: This has been split into the two named parameters preferEphemeral (for iOS and MacOS) and intentFlags (for Android) within FlutterWebAuth2Options. The former works exactly the same. However, if you want the old behaviour using preferEphemeral on Android, use the ephemeralIntentFlags constant as value for intentFlags.
Requires an update to flutter_web_auth_2 3.1.1 as per their docs: https://pub.dev/packages/flutter_web_auth_2