It's been a while since Windows Phone 8.1 received an update; indeed, the last one before the Windows 10 Mobile launch was Lumia Denim. Even when Windows 10 did come to older phones, it required the user to know about it, download an app from the Store, and explicitly opt into the upgrade.
While it's been almost 15 months since Windows 10 Mobile was made available, you can still upgrade your older handset today. That could change in four weeks, as mainstream support for Windows Phone 8.1 officially ends on July 11.
Microsoft includes the following notes:
Microsoft will make updates available for the Operating System, including security updates, for a minimum of 36 months after the lifecycle start date. These updates will be incremental, with each update built on the update that preceded it. Customers need to install each update in order to remain supported. The distribution of these incremental updates may be controlled by the mobile operator or the phone manufacturer from which you purchased your phone, and installation will require that your phone have any prior updates. Update availability will also vary by country, region, and hardware capabilities.
To be clear, the Lifecycle Start Date was June 24, 2014, and there is no extended support date listed.
Being that it's been so long since Windows Phone 8.1 received an update at all, it was already fair to assume that it wasn't getting more of them. Still, the description implies that as of that date, Microsoft is no longer promising to make existing updates available to those that haven't installed them yet.
In other words, if you were putting off upgrading your older handset to Windows 10 Mobile, now is the time to do so. We've reached out to Microsoft to try and confirm whether or not it will still offer upgrades after July 11, and we'll let you know if we receive a response.
As a reminder, last time this happened was with now-unsupported devices that used to be able to get Windows 10 Mobile via the Insider Preview, and while we tried to get confirmation on that ahead of time, Microsoft shut down that upgrade path without warning.