Isso é o que diz na especificação do Windows 10
For anti-malware applications, Windows will check to see if your anti-malware subscription is current (not expired) and compatible during the upgrade. If the anti-malware application is compatible and current, your application will be preserved during the upgrade to Windows 10. If the anti-malware application is incompatible, Windows will uninstall your application while preserving your settings. After upgrade is complete, if your anti-malware provider has informed Microsoft that it has made a compatible version available for your active subscription, Windows will notify you to install the latest version available with the settings that were set prior to upgrade. If your anti-malware subscription is not current (expired), Windows will uninstall your application and enable Windows Defender. Source
Portanto, é possível que tenha sido desinstalado.