This apparently only affects "New Outlook", Outlook on the Web, and will soon be on Outlook for iOS, Outlook for Android, and New Outlook for Mac Users. If you're using the New Outlook, switch back to the 'regular' Outlook. New Outlook is basically OWA for desktop and you're going to lose a lot of the features offered in the classic Outlook Client. New Outlook is free. That should tell you something right there.
With classic Outlook, you won't have this problem. Yet. I'm still trying to see how to turn off the banner in the New Outlook client from the Exchange Online/Defender configurations. A teammate was told by a Microsoft tech that there was no way to turn this off. I agree, it's total ****. Personal email users may need some reminders and this banner, but in a corporate environment, with a third-party service (Proofpoint) that adds its own banner, we don't need TWO banners on our messages.
Microsoft, cut the ****.