When Windows 7 reaches end of support, the operating system will display a warning stating that Windows is more vulnerable to viruses and that you should upgrade to Windows 10. Microsoft strongly recommends using Windows 10 on a new PC for latest security features and protection against malicious software. Users can configure the following registry key before January 15th, 2020 in order to never see the alert. Windows 7 Professional customers who have purchased Extended Security Updates, domain-joined machines, or machines in kiosk mode will not be shown this message.
Source: https://www.bleepingcomputer.com/news/microsoft/windows-7-to-show-full-screen-windows-10-upgrade-alerts/