Hello Windows Insiders,

No doubt there are questions about the information shared today covering Windows 10 Mobile and how various Windows Phone 8.1 devices are affected. This post is to address questions that you may have and to share details on some of the decisions that were made.
Early today Gabe posted in the Insider Hub:

Thank you to Windows Insiders and the availability of Windows 10 Mobile.
Today we announced the availability of the Windows 10 Mobile upgrade to select Windows 8.1 phones on the Windows Blog here. We are grateful to all of you who choose to install Windows 10 Insider Preview builds on your own phones and provide us feedback. The first mobile build was sent out to Windows Insiders just over a year ago on Feb 12th, 2015. Thank you to all of you for helping to make the Windows 10 mobile launch today possible.

Today’s roll-out to a subset of Windows 8.1 devices is based on the customer experience we have heard from you, approvals from device manufacturers, and validation by mobile operators. We also have heard through feedback from Windows Insiders that many older Windows Phone devices did not meet our expectations for quality in experience with Windows 10 preview builds, and that feedback helped us choose which devices would receive the update to Windows 10.

As we stated previously, devices which are eligible to receive Windows 10 will also be able to get preview builds from the Development branch. We’ve already delivered a few builds to the devices which shipped with Windows 10, including the Lumia 550, 650, 950, 950 XL, Xiaomi Mi4, and Alcatel OneTouch Fierce XL. We have updated the list of devices that will be capable of receiving updates as part of the Windows Insider Program going forward. Devices on this list that opt in to the Fast or Slow rings, will begin to receive new Windows 10 Insider Preview builds from the Development Branch in the coming weeks. Windows Insiders who opt in to the Release Preview ring will receive app and servicing updates from the Current Branch earlier than other customers, but will not receive in development Windows 10 Insider Preview builds.

Many thanks,

One of the key things Insiders have done over the past year of mobile flighting has been providing feedback. We’ve received your feedback covering every aspect of the device experience; this includes OS layout and features, speed and responsiveness of devices, OS stability (freezing/crashing), as well as general usability. This feedback was invaluable in guiding the decision on which devices would benefit from an update to Windows 10. In reviewing all available information and feedback, not every device proved to be a good candidate for this update. As a result, not all devices will be targeted to receive an official Windows 10 update.

Our goal is to only offer the Windows 10 upgrade to devices that we are confident can continue to deliver a great customer experience.

Q: Which devices will the official Windows 10 upgrade be available to?
A: You can see the list of Windows 8.1 phones that may be able to upgrade to Windows 10 here. To download the Windows 10 advisor app, go here.

Q: If I already have the current Windows 10 Mobile Insider Preview build and my device is on the updated list of supported devices for the Windows Insider Program, do I need to do anything?
A: No action is required on your part. As a Windows Insider you have the latest build and do not need to downgrade to Windows Phone 8.1 to receive any updates. Your device is up-to-date. If you no longer want to receive Windows Insider Preview builds on a particular supported device, you may switch to Production and await future retail updates or move to the Release Preview ring to receive early app updates.

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: What does this mean for Windows 10 Mobile builds from the Development Branch?
A: Currently builds from the development branch have been made available to the Lumia 950, 950 XL, 650, 550, Xiaomi Mi4, and Alcatel OneTouch Fierce XL. In the coming weeks we will be making future Windows 10 Insider Preview builds available to devices on the updated Windows 10 Mobile device list. We will add devices to the Windows Insider Program as more devices launch with Windows 10 Mobile.

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.

We know this news will be exciting for some Insiders and disappointing for others. The decisions on which devices to proceed with were made after extensive analysis of Insider feedback. We truly appreciate the time and effort our Windows Insiders have provided to help ship Windows 10 Mobile.

There may be additional questions that arise and we will update this post as necessary. If there are any outstanding questions, please share so we may address them.

Thank you as always,

Source: Windows 10 Mobile Insider Device Rollout FAQ - Microsoft Community