Windows Defender ATP security support is now available now for WIN7 and WIN8.1 as documented below

The Windows Defender Advanced Threat Protection (ATP) service can now be used to help address security issues with Windows 7 and Windows 8.1 clients.  The capability for Windows Defender ATP to act as an “endpoint detection and response” service for those older Windows operating systems reached “general availability” status, Microsoft announced on Friday. It means that organizations can now use the service in their production environments to track Windows 7 and Windows 8.1 security issues. The capability was at preview stage last year.