Starting with the Windows 10 Creators Update, Windows Defender Antivirus always pops a toast notification for recent activities and scan results, and if you the notification annoying, there is a quick way to disable the feature.
Open Windows Defender Security Center.
Click the Settings (gear) button at the bottom of the app.
Turn off the Receive recent activity and scan results toggle switch.
Here's a look at the notification, I'm referring to.
After completing the steps, moving forward, you'll no longer see notifications from Windows Defender Antivirus.