Windows 10, version 1803, also known as the April 2018 Update, has now reached end of service for Home, Pro, Pro Education, and Pro for Workstations editions. Microsoft might still release security updates for critical vulnerabilities that could potentially have a major impact on no longer serviced Windows versions such as Windows 10 1803. Windows Update will automatically initiate a feature update to May 2019 Update to allow users to continue receiving updates. Not all features in an update will work on all devices, as Microsoft’s Windows lifecycle fact sheet notes.
Source: https://www.bleepingcomputer.com/news/microsoft/microsoft-auto-updating-windows-10-1803-devices-to-may-2019-update/