Sony's concept updates are a bit of a wild card in that they serve perfectly functional versions of pure Android but aren't really meant as your daily driver. Rather, they are a way for Sony to test features and get user feedback before implementing them in the official firmware.
This way, Sony users get a Nexus-like experience while helping the company do due diligence, which is a win for both sides. But Sony has no interest in maintaining Concept update programs longer than it has to, which is why the Xperia X is getting axed out of the program. Users of the official firmware need worry not, though. Sony says it's going to push security updates "for a fair bit longer."
The Xperia X was released in May last year and has a 5-inch 1080p display. It's powered by the Snapdragon 650 processor and comes with 3GB of RAM, 32GB of storage, and a 23MP camera.
source: Xperia Blog