With the arrival of Windows 10, the Serving plan was introduced. This indicates which versions (builds) of Windows are still supported by Microsoft. No updates will be released for older Windows versions. No security updates either.
In the future, Microsoft will no longer release newer Windows version numbers as in the past with Windows 7 and 8, but newer builds of Windows 10 will be released. In fact, we can no longer call it Windows 10, but only Windows. Without a version number.
Release ready: 1709
Business Ready: 1703
End of life: 1507