https://www.bleepingcomputer.com/news/microsoft/microsoft-resumes-delivering-windows-7-defender-definition-updates-after-2-weeks/

Microsoft appears to have fixed an error in the distribution of Windows Defender definition updates on Windows 7 because the older OS is now receiving signature updates once more.

The problem started manifesting around June 11, when German blogger Günter Born noticed that Windows Defender on Windows 7 systems wasn't receiving automatic definition updates via the OS' built-in Windows Update mechanism.