Microsoft formally announced back in May that the offer for a free upgrade to Windows 10 will expire on July 29, exactly one year from launch as promised. There has been some question since then if this would also apply to devices that are eligible for Windows 10 Mobile.
After all, Microsoft hasn't done much to push Windows Phone 8.1 users into upgrading. Users need to have the Upgrade Advisor app and for that to happen, they have to know it exists, for which there is no indication.
Dona Sarkar took to Twitter today to confirm that indeed, Windows Phone 8.1 users will still have the option of upgrading to Windows 10 Mobile after the PC deadline expires.
Of course, the question is still unanswered of what will happen to unsupported devices that wish to use Windows 10 Mobile. Currently, there is a sort of "upgrade at your own risk" option for them, which involves signing up for the Release Preview ring of the Insider Preview. When the Windows 10 Anniversary Update is released, it will be on the Release Preview ring, so it's unknown if build 10586 will continue to be available for those devices.
Microsoft has also confirmed to Neowin that those using unsupported devices will not be able to continue getting new builds on the Release Preview ring. They have been supported thus far with cumulative updates, but they will not be able to receive Redstone builds through this method.