Chris Hajer: https://github.com/gravityforms/support-backlog/issues/999 - this was not in 2.9.12. Publish when released in an interim release or 2.9.13
Description
This constant enables the X-Gravity-Forms-Source email header for notification emails.
Usage
define( 'GF_ENABLE_NOTIFICATION_EMAIL_HEADER', true );
Placement
This constant should be set in your wp-config.php. See the article WP Config Options for more details.
Source Code
This constant is located in common.php
Since
This filter was added in Gravity Forms 2.9.13