Sony may have dropped Xperia Z1 out of its Marshmallow update release plans, but the company will continue to support the device through its AOSP project, where they post binaries for the latest release of Android for various Xperia devices. This makes it easier for developers to build AOSP based ROMs.
Sony released Android 6.0 Marshmallow binaries yesterday for all the supported Xperia devices under the AOSP project, and it seems Xperia Z1 and Z Ultra have already received Marshmallow ROMs based on Sony’s binaries.
Developer Ev3rtH over at xda has posted fastboot flashable Marshmallow ROM for Xperia Z1 and Z Ultra. However, as is the case with first releases of newer AOSP ROMs, Ev3rtH’s Marshmallow ROMs for both the devices are also pretty buggy at the moment.
Also, the Android 6.0 Marshmallow Gapps that we tested to be working on Nexus devices and other phones with AOSP ROMs, doesn’t seem to work with Xperia Z1 and Z Ultra Marshmallow ROMs.
We wouldn’t recommend you to install these early releases of AOSP based Marshmallow ROM for Xperia Z1 and Z Ultra until Gapps is fixed. Because you won’t have Play Store and other Google services without installing a compatible Gapps package.
You can track development of the ROM from the original development threads linked below for both the devices:
We’ll update this page once the ROMs are stable enough to be used as daily driver. Until then, only the adventurous ones and the people who would like test and help with the development should install the ROM on their devices.