Those Windows Insiders who are now on Windows 10 Mobile upgrade unsupported devices are in a strange situation. They are already on latest Windows 10 Mobile Build 10586.164, but since there devices are not supported for Insider program by Microsoft anymore, they can’t continue to Windows 10 Mobile Insider program too. So, here is what Microsoft has to say about your situation.

Q:  I’m currently using a Windows 10 Insider build on my device, but it is not on the list of devices that will receive a formal Windows 10 update.  What should I expect in the future?
A:  If you already have the Windows 10 Mobile Insider Preview build on your device, you may keep using your device in an unsupported state. We will not be offering build updates beyond the Current Branch build 10586. At some point in the future these devices will also stop receiving updates to the Windows 10 Mobile Insider Preview build.

In order to recover the devices back to a supported build of Windows Phone 8.1, you may use the Windows Device Recovery Tool. There are more details on the use of the Windows Device Recovery Tool within this forum post.

Q: Why are you choosing to remove devices from the Windows Insider Program?
A: By limiting participation in the Windows Insider Program to current devices that have an official upgrade to Windows 10, or new devices that are shipping with Windows 10, this helps focus investigating feedback in the Development Branch for devices that will receive future Windows 10 updates.