The Microsoft Lifecycle policy page reveals that Windows 10 Mobile version 1511 and Windows 8.1 (Mainstream support) have reached the end of support status on January 9, 2018. In case of Windows 10 Mobile version 1511 while it is the complete end of support, for Windows 8.1 it means exiting mainstream support and entering the extended support.
The latest Windows 10 Mobile version 1511 build number is Build 10586.1356. You need to update to a newer Windows 10 Mobile OS version like version 1607 (Anniversary update), version 1703 (Creators update) and version 1709 (Fall Creators update) for remaining supported.
You can go to your settings and check for the update to update to a latest Windows 10 Mobile version depending upon your phone. You can also use the Windows device recovery tool to check for available updates and upgrade.
Coming to Windows 8.1, yesterday it exited the mainstream support and entered the extended support. Which means Microsoft will support Windows 8.1 with only security fixes going ahead until October 2023. You can still upgrade to Windows 10 for free by using a convenient loophole provided by Microsoft.