https://www.theregister.co.uk/2019/04/11/windows_19h2_missing/

Fast Ringers keen to extricate themselves from the Windows Insider programme have missed their chance for a quieter life as Microsoft booted them into the 2020s with build 18875.

Before today, Microsoft's platoon of unpaid Fast Ring testers could have switched away without needing a reinstall, but no more. Like the vampire of yore, if the 20H1 build is allowed over the threshold, there will be no going back without a stake to the heart of your PC.