Those Lumia devices were among the last to ship with Windows 10 Mobile, and while they still "work" per se, they aren't being updated with new features and aren't nearly as capable as the Android and iOS peers. So, what is someone to do with an old Lumia 950 XL that they may have lying around? Install Windows 10 on ARM, of course!
Software developer Gustave Monce has been working on bringing the full-blown Windows 10 on ARM experience to Lumia devices for quite a while, and he's now providing firmware images which allows Lumia 950 and Lumia 950 XL owners to relatively easily get the operating system onto their smartphone.
Windows 10 on ARM running on even the Lumia 950 XL with its 5.7-inch display is going to be a bit of a cumbersome experience, and we're not so certain how the operating system is going to perform on the Qualcomm Snapdragon 810 processor (circa 2015). But it's at least worth experimenting with if you like a fresh challenge.
There are some drawbacks, however, to installing Windows 10 on ARM onto your Lumia device. For starters, it will completely wipe out your existing Windows 10 Mobile installation, so you'll want to make a full backup of all of your data. In addition, there are currently no provisions for dual-booting with Windows 10 Mobile and Windows 10 on ARM.
In addition, we don't see any references to being able to retain the ability to make phone calls (although someone please correct us if we're wrong on that one).