the executable which shows the the Windows Defender Notification icon in systray or notification area is %ProgramFiles%\Windows Defender\MSASCuiL.exe.
This program is simply started by a run key in HKEY_LOCAL_MACHINE. So if you want that the icon is gone and no longer being shown delete the run key.
C:\> reg delete HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Run /v SecurityHealth /f
If you want it back recreate the key
C:\> reg add HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Run /v SecurityHealth /t REG_SZ /d "%ProgramFiles%\Windows Defender\MSASCuiL.exe" /f