A common way to install Windows 10 for phones preview on devices that are not on the supported list, up until now, was to use a spoofer such as the Fiddler. Fiddler would allow you to trick Windows Insider app into thinking that you are using one of the “allowed” mid-rage Lumias, even if you’re not, and the preview OS would install.
Microsoft has good reasons for allowing the new OS to be installed only on some devices (for now) but many folks wanted to take the risk anyway, and try it on an unsupported devices. This has occasionally led to big issues for some of these users and Microsoft just doesn’t want to deal with helping folks revert back to WP 8.1 when they should not have installed Windows 10 to begin with.
With that being said, Windows Insiders app has been updated today to version 126.96.36.199 and many users are reporting that they can no longer spoof their device with this update. So it does look like Microsoft has indeed found a way to prevent this sort of thing form happening but beside that, there are likely other updates bundled. There is no official change log available.
So head to the Windows Phone Store and grab this update as it is highly recommended. We are very certain Microsoft will release Windows 10 preview to more devices pretty soon, it is worth the wait to not have your phone bricked for doing something that is not recommended nor supported.