For reCaptcha to work, the server must be configured to access URLs on the Internet. It doesn't appear this is tested as a requirement when installing phpBB and it's not a showstopper. But if it isn't available, the reCaptcha server can't be reached and its use will fail, leading to admins to change the countermeasure and to people complaining they can't register. So I suggest disabling it as an option if the setting is not enabled and/or putting up some sort of warning message that allow_url_fopen must be set to On if it is off.