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".
When the banner is clicked, it will open the default browser to the https://www.microsoft.com/en-us/windows/comprehensive-security/ web page. This page explains that it is important for users of older Windows 10 versions to update to the newest version in order to have the latest security updates and features available to stay protected.
"Windows 10 delivers comprehensive, built-in and ongoing security protections you can trust – including Windows Defender Antivirus, firewall, and more," the Microsoft page states. "By staying up to date, you’re assured of the latest features and protection – at no extra cost to you."
The reason why they are giving October 9, 2018 as the date to update by is because that is the day that Microsoft will no longer support the Windows 10 Anniversary Update. That means that users of this version will no longer receive security updates that are released and thus will no longer be protected from newly discovered threats.
Users of other versions of Windows 10 will continue to receive security updates until their support ends. For users of the Creators Update this will be on April 9th 2019, for Fall Creators Update it will be on October 8, 2019, and for the latest April 2018 update, it will be to November 12, 2019.