Installing software updates is kinda like flossing: Many people choose not to despite knowing they probably should. But there is one important difference: dentists can't force people to floss. Developers can push updates to their users, though, and Microsoft exercised that option yesterday by starting automatic updates for Windows 10 users on the April 2018 Update (1803) to the May 2019 Update (1903).
Microsoft will end support for the Home and Pro versions of the Windows 10 April 2018 Update on November 12. (Support for the Education, Enterprise, and IoT Enterprise versions will continue until November 2020.) Anyone still using that version of Windows 10 after that date will have to accept the risks--security and otherwise--associated with relying on an unsupported operating system.
The problem is that many people are still using Windows 10 1803. That's probably at least partly because the following release, the Windows 10 October 2018 Update (1809), was plagued by technical issues that led to several delays. Even after it was officially re-re-released in January, subsequent cumulative updates led to various performance issues. Waiting out those problems seemed wise.
But using an outdated version of Windows 10 (or any software) also has its drawbacks. Vulnerabilities discovered in an older release of the operating system, for example, might not affect the current version. People who continue to use Windows 10 1803 aren't benefiting from any of the security updates included with versions 1809 and 1903. Microsoft explained in a support article on the automatic updates:
"We are initiating the Windows 10 May 2019 Update for customers with devices that are at or nearing end of service and have not yet updated their device. Keeping these devices both supported and receiving monthly updates is critical to device security and ecosystem health. Based on the large number of devices running the April 2018 Update, that will reach the end of 18 months of service on November 12, 2019, we are starting the update process now for Home and Pro editions to help ensure adequate time for a smooth update process."
The company announced that it would use machine learning to update Windows 10 April 2018 Update users to the Windows 10 April 2019 Update in June. It's not clear what caused the delay, but it actually started to push those updates on July 16. Manual updates are available via Windows Update for most systems; Microsoft will prevent some from installing the update because it fears they aren't compatible.