Sony's back to its AOSP tricks, working to release some functional (if not exactly ideal) versions of the latest release of Android based on open-source code. This time they've quickly put together Android Open Source Project builds for the flagships of the last two years: the Xperia Z1, Z2, and Z3. You can see the bone-stock builds running in the video below.
As always with Sony's developer promotions, these builds aren't intended for end users - they aren't provided with any kind of promise for reliability or functionality. They don't even include the Play Store or other standard apps, which many people (erroneously) think of as part of "stock" Android. The builds are provided more or less as a starting point for custom ROMs. It helps, too - Sony hardware is disproportionately featured on the CyanogenMod download page, and a liberal attitude towards bootloaders means that most Sony phones see more aftermarket development than other manufacturers.
Right now the updated binaries for the Xperia phones haven't been posted, so you probably can't compile a version of AOKP running on a Z1, Z2, or Z3 even if you want to. But according to the Sony blog post, Sony will post those soon enough, probably along with the source code needed to build the software yourself. There's still no indication that Sony is interested in releasing consumer-ready stock Android builds... but we can dream.
Source: Sony Mobile Blog