While Microsoft may have already ceased updates for Nokia X family of devices, the first Nokia branded devices to run Android, the developer and enthusiast community is trying to bring custom ROMs based on latest Android versions like Marshmallow to the devices.
On this page we have covered Nokia X2 Cyanogen
So, you have liked everything about Lumia 950 and Lumia 950 XL, but the polycarbonate body design. So, now you can get your Lumia 950 XL with all the goodies that it packs even in Metal and Leather. This is possible because of third party back covers from "Mozo Accessories".
Lewa was the first one to release a custom ROM for Nokia X in very early days of the device launch. Now since Microsoft has done away with software development for Nokia X series of devices there is no possibility for any new OS / Firmware coming officially for Nokia