Mainstream support has ended for Windows 10 Mobile version 1511. Both consumer and Enterprise releases are affected, according to Microsoft's product lifecycle page, meaning that no more cumulative updates will be delivered to devices remaining on the original release version of the mobile OS (via Neowin).
Windows 10 Mobile version 1511 originally shipped in November of 2015 on the Lumia 950 and 950 XL, but it didn't roll out to Windows Phone 8.1 devices until months later. Even among those currently sticking with a Windows 10 Mobile device, dropping support for version 1511 is unlikely to affect many. According to the latest available numbers from AdDuplex, released in June of 2017, Windows 10 Mobile version 1511 was installed on 4.6 percent of all Windows 10 Mobile phones.
If you're running a phone on the Anniversary Update or later, you'll continue to be supported going forward. Support for the most recent version, released in October of 2017, is scheduled to end in December of 2019. As for Windows 10 Mobile itself, well, it's not much of a focus for Microsoft anymore.