With the Windows 10 Redstone build, Windows Defender icon is added to the notification area and can’t be removed via Settings. However, you may remove the icon by disabling an optional startup entry added by Windows Defender.
The Anniversary Build (or Redstone 1) includes an additional executable named MSASCuiL.exe which adds the Windows Defender tray icon.
Remove Windows Defender Notification area icon
To remove Defender icon from the notification area (effected from next login), stop this program from loading at startup.
Right-click Taskbar, click Task Manager and select the Startup tab.
Right-click Windows Defender notification icon, and click Disable
With Defender’s notification icon program disabled, you can still launch and use Windows Defender as usual, via Settings > Update & Security > Windows Defender.
Windows Defender Notification area icon appears again?
Although the Windows Defender Notification area entry is disabled from loading at startup, launching Windows Defender program window (msascui.exe) would also launch msascuil.exe which shows the notification icon. The icon would remain in the notification area even after you close Windows Defender. Also, any attempts to block msascuil.exe would cause considerable delay when starting Windows Defender user interface, and the window may stop responding for a few moments.
To remove the icon, logoff and login back or kill the process manually using Task Manager.
Or, run the command “tskill msascuil” from Run dialog (WinKey + R).
Note: This doesn’t disable Windows Defender’s real-time protection feature. Real-time scanning is taken care of by the Windows Defender service and core modules, and the main Windows Defender user interface is MSASCui.exe.