It is not possible to install Windows 10 on iPhone X or any other custom operating system. Apple disallows other operating systems by using bootloader defense mechanisms. However, iPhone can only use the OS signed by Apple itself. Although Windows 10 can’t be installed on your iPhone X you can use iOS Community Custom Firmware for Jailbroken phone. It is the reason why you cannot downgrade the current operating system.
About Windows 10
Windows 10 is one of the family members of Operating systems developed by Microsoft. It is the latest development by Microsoft by being the successor of Windows 8.1 which was released on July 15, 2015. This operating system has got many positive reviews and is being praised for its design and features.