As pre-release software, Insider Preview builds have a built-in expiration date that we call a “timebomb”. This has been included ever since the first Windows 10 Insider Preview build and helps ensure Insiders stay up to date with the latest features and quality updates. Once a build expires, Insiders will be provided a warning that the build has expired and will get that warning once a day. In addition to that, Insiders will also see User Access Control (UAC) warnings as well. Insiders in both the Fast and Slow rings need to make sure they are running the latest builds offered to those rings. Insiders in the Fast ring need to make sure they take the update to 19H1 Build 18290 (today’s flight) in order to avoid hitting the timebomb set to Dec. 14th . Additionally, Insiders who used ISO for Build 18272 also need to take the Build 18290 flight to avoid hitting the timebomb. We will be removing these ISOs from the download page shortly. Insiders in the Slow ring running older builds need to update to Build 17763 (the final build for the October 2018 Update) to avoid hitting the timebomb in older RS5 builds. Insiders can check their flight settings and verify they have the latest builds by going to Settings > Update & Security > Windows Insider Program.
TIP: You can check the build expiry date of your Insider Preview build at any time by typing WINVER in search on the taskbar.