This is what Microsoft says.

Yes, we recommend all Windows devices use antivirus software. Currently, the only antivirus software known to be compatible with Windows 10 in S mode is the version that comes with it: Windows Defender Security Center. Windows Defender Security Center delivers a robust suite of security features that help keep you safe for the supported lifetime of your Windows 10 device. For more info, see Windows 10 security.

Also note that by exclusively using apps from the Microsoft Store and browsing safely with Microsoft Edge, Windows 10 in S mode keeps you running more quickly and securely day in and day out.

https://support.microsoft.com/en-us/help/4020089/windows-10-in-s-mode-faq