https://www.bleepingcomputer.com/news/microsoft/microsoft-rolls-out-windows-10-1903-on-april-2018-update-devices/

Microsoft says that Windows 10 version 1803 devices reaching the end of 18 months of service will start receiving the Windows 10 May 2019 Update as part of an effort to keep them well serviced and in a secure state.

"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," says Microsoft.