Spooky timing, this email from Microsoft has just landed...
Hi Windows Insider, Some PCs in the Dev Channel do not have an option to switch channels as they were given an exception to install Windows 11 preview builds as outlined in our blog post here on June 24th (the red column in the chart). These PCs do not meet the minimum hardware requirements for previewing Windows 11 builds and will not receive new Windows 11 Insider Preview builds from the Dev Channel going forward. Based on our data, you have a PC that was given that exception. This PC must clean install back to Windows 10 with the media (ISOs) that we provide and can then join the Release Preview Channel to preview Windows 10 updates. Please review the message in your Windows Update Settings to learn more.