The Inquirer writes users of Windows 10 Spring 2018 Update will receive a window to update the operating system. The upgrade message can be deferred for a maximum of 35 days, after that it will not be possible to further delay the upgrade.
It appears quite a lot of machines are still running Windows 10 Spring 2018 Update:
As Microsoft explains: "Keeping these devices both supported and receiving monthly updates is critical to device security and ecosystem health."
If it hadn't been for the blessed 1809, this would probably not be news, but given that it has been suggested that less than a quarter of machines have made the update from 1803, this is going to affect a lot of users:
"Based on the large number of devices running the April 2018 Update, that will reach the end of 18 months of service on November 12, 2019, we are starting the update process now for Home and Pro editions to help ensure adequate time for a smooth update process."