Windows 10 1909 feature updates will start automatically installing on Windows 10 1809 devices beginning today. Windows 10, version 1809 was made available more than a year ago, on November 13, 2018, and it has an end of service date of May 12, 2020. Microsoft might still issue updates for highly critical security vulnerabilities that may potentially have a major impact on no longer serviced versions such as Windows 1809. Users might still be able to defer the automatically initiated feature update on their outdated Windows 10 devices with the help of advanced update settings.
Source: https://www.bleepingcomputer.com/news/microsoft/microsoft-starts-forced-feature-updates-on-windows-10-1809/