Disable Warning Banners in Google Workspace

To prevent banners from appearing on Phishing Simulation emails, go to Apps > Google Workspace > Settings for Gmail > Spam, Phishing, and Malware > Add Another Rule. Under "Bypass spam filters for messages from senders or domain in selected lists" click "Create or edit list" and add all domains used for phishing simulation testing. 

Warning banners can be turned on or off at the domain and OU level. Visit the Help Center to learn more about how to add custom spam filters to Gmail.