https://wccftech.com/windows-10-1903-forced-upgrade-to-begin-soon-for-version-1803-users/

Microsoft promised to improve the Windows 10 update system by giving users control over when their devices get updated to the latest version. The new system allows users to delay updates for up to 35 days. However, the Windows maker had also said that the company will force update machines when their currently running OS version is going to meet its end of support deadline.

For Windows 10 April 2018 Update, that deadline falls on November 12. Microsoft had earlier noted that it will “begin updating devices running the April 2018 Update and earlier versions of Windows 10 in late June 2019 to help ensure that we keep these devices in a serviced, secure state.”