We have updated the WordPress core upstreams (WordPress and WordPress (Composer Managed)) to provide a new PANTHEON_HOSTNAME constant. This value can be helpful when defining DOMAIN_CURRENT_SITE on WordPress multisite installations. By default, the PANTHEON_HOSTNAME constant is set to the value of the HTTP_HOST server variable, which is the hostname of the request. However, when this value is unavailable, the PANTHEON_HOSTNAME provides fallback values, thereby avoiding "Undefined index: HTTP_HOST" warnings. For more information, refer to our WordPress Multisite configuration guide.
The latest 1.4.1 release of the Pantheon MU Plugin updates the recommended configuration for WordPress multisite to use the new PANTHEON_HOSTNAME constant when defining DOMAIN_CURRENT_SITE rather than the previous recommendation -- either $_SERVER['HTTP_HOST'] or a complicated PHP switch.