Who says updates for the Xperia M were dead? After the Xperia M received a new firmware certification back in September for build number 15.4.A.1.10, the update has finally started to roll. However, this might be a carrier-only update – it has only hit Boost Mobile in Australia so far.
This is a very small update, moving the build number from 15.4.A.1.9 to 15.4.A.1.10, so only expect some small bug-fixes. It is currently rolling as a 20.1 MB OTA update, but if you want to try it for yourself you will find the FTF file below for the C1904 Xperia M variant.