When Microsoft stops supporting a version of Windows, Microsoft stops issuing security updates for that operating system. For example, Windows Vista and Windows XP no longer receive security updates, even if substantial security holes are found in them. On January 14, 2020, the same will be true for Windows 7.
https://support.microsoft.com/en-us/help/4057281/windows-7-support-will-end-on-january-14-2020