https://www.bleepingcomputer.com/news/microsoft/microsoft-store-is-prompting-older-windows-10-versions-to-update/

The Microsoft Store has started to display banners that prompt users who are running older versions of Windows 10 to update to the latest April 2018 Update. This banner is being displayed to all Windows 10 users who are not using the latest feature update.

As shown above, this notification comes in the form of a large banner that states "Update your PC by October 9, 2018. Join the millions of people who are up to date".