Launched in 2009, I think it’s only the Android handset to get ported with Android 4.4 Kitkat firmware. No chance that it will be official firmware as Sony has no plans to come out with one such for any of the Sony Ericsson Xperia phone after their partnership ended in 2011, so this should be a custom firmware and yes indeed.
The Sony Ericsson Xperia X10 Kitkat firmware via unofficial CM11 is now available and it has been ported by XDA Forum senior member named Tof37. Still this Android 4.4 Kitkat CM11 built is alpha 1 stage and naturally many things might not work on your phone. With Sony Ericsson came up with locked bootloader, this device receives some serious love from the developer. Seriously that’s a good amount of hardwork to come up with Android Kitkat after missing out all the Jelly Bean update.
If you happen to own Sony Ericsson Xperia X10 smartphone and wanted to breathe in a new life to your smartphone then make sure to check out XDA-Thread link to check out on how you can update your device. Let us know what doesn't work and what works after updating your device.