Windows 10 on ARM shown running on a Lumia 1520
One of the things that Windows Phone, and by extension Windows 10 Mobile, was known for is its high level of security. Unlike Android, developers on Windows 10 Mobile struggled a lot to get root access and unlock the bootloader of the device. All that changed with the release of WP Internals 2.3 by XDA developer Heathcliff74, which allows any Lumia phone to be rooted and bootloader unlocked. Now, other developers are reaping the fruits of Heathcliff74's efforts by trying to run Windows RT and even Windows 10 on ARM on older Lumia devices such as the Lumia 1520.
Gustave M. (@gus33000) was successful in getting the now defunct Windows RT to run on a Lumia 1520. He said that getting to run Windows RT on newer Lumias is easier than doing the same on older Lumia x20 devices and this feat would not have been possible if not for the WP Internals tool. Another developer, Blaze, was also successful in getting Windows RT to run on the Lumia 950. Gustave then took things a notch higher by getting Windows 10 on ARM (Build 17061) to run on the Lumia 1520 and he was successful again. He, however, noted that there are glitches and the touchscreen was not working due to lack of proper drivers. Nevertheless, the big picture here is that even if Microsoft has abandoned erstwhile Lumia loyalists, the developer community is taking things into their own hands and trying to develop some cool hacks. Heathcliff74 says that his tool has been downloaded more than 10,000 times in the past few days.
Gustave has not documented his installation process but he might possibly do so once he gets some of the larger issues resolved. Hopefully, other enterprising developers will soon come up with a way to install a more feature-filled version on Windows 10 and custom ROMs on the ageing but still capable Lumia devices.