Today, Microsoft is ending support for the original version of Windows 10 Mobile, version 1511, as well as the same version of Windows 10 Mobile Enterprise. This means that there will be no more cumulative updates for those that are still using that version of the OS. Of course, there hasn't been a cumulative update for version 1511 for some time, although it's still possible that Microsoft could release one final update as part of this month's Patch Tuesday builds.
As of the AdDuplex report from last June, the last to report on Windows 10 Mobile versions, just 4.6% of Mobile devices are on 1511, and 20.3% of all Windows phones are on Windows 10 Mobile. Mainly, this should consist of devices that signed up for the Windows 10 Insider Preview before Microsoft ended support for them (old Windows Phone 8 devices and so on), as well as the few devices that were supported, but never got the Anniversary Update, such as the Acer Liquid Jade Primo.
Version 1511 is listed as having shipped on November 16, 2015, so it's well over two years old. This, however, refers to it shipping on new devices, such as the Microsoft Lumia 950 and 950 XL. If you had a Windows Phone 8.1 device and waited for the official upgrade - instead of signing up for the Insider Program - it wasn't actually released until March 17, 2016.
If your phone is eligible for a newer Windows 10 Mobile version and you've somehow not updated your device yet, it's recommended that you do so by going to Settings -> Update & security -> Phone update -> Check for updates. Version 1607, or the Anniversary Update, is still being actively supported with cumulative updates.