When Microsoft released the Windows 10 Mobile upgrade back in March, it was available to a fairly short list of devices compared to the promise of all devices running Windows Phone 8.1. Those unsupported devices, however, were offered a workaround by upgrading through the Release Preview ring of the Insider Preview.
Microsoft announced today that the workaround will no longer work. If you have an unsupported device and you're on Windows 10 Mobile, you won't receive any updates beyond build 10586. If you use the Windows Device Recovery Tool to roll back to Windows Phone 8.1, you will not have the option to upgrade your device back to Windows 10 Mobile.
Of course, there is a reason for this. The Release Preview ring was designed to offer early access to cumulative updates, or anything that non-Insiders are going to eventually get their hands on. Microsoft allowed unsupported devices to continue getting the 10586 builds, since they were supported during the Preview period. Those devices were never supported for any of the Anniversary Update previews, so now that the Anniversary Update is in the Release Preview ring, 10586 is not.
Microsoft has been very clear that unsupported devices would not have a path beyond 10586 since the very beginning; however, the company did not, and would not, say whether build 10586 would continue to be available once the Anniversary Update was.
In fact, we've been trying to ask Microsoft about this for weeks, hoping to offer users a "last chance" to upgrade, rather than having the opportunity taken away without warning.
For those that are already on Windows 10 Mobile with an unsupported device, we've reached out to Microsoft to confirm whether or not there will be any future cumulative updates for build 10586. Clearly, the probable answer is no, but Windows 10 for PCs will most certainly have updates for the build (businesses need them; in fact, there are still 10240 updates rolling out), and since Windows 10 is developed in parallel with Windows 10 Mobile, anything is possible.
Nevertheless, if you're on an unsupported device, you no longer have an upgrade path to Windows 10 Mobile. However, if your Windows phone doesn't use a Snapdragon S4 chipset, has 1GB RAM or more, and 8GB of internal storage or more, there's always a chance that it can be added to support, much like the Yezz Billy 4.7 was.