CONFIG_UNUSED_BOARD_FILES is not available for thedefaultarchitecture x86.
Result is shown for architecture arm
default or selected kernelversion does not have config value CONFIG_UNUSED_BOARD_FILES.
Result is shown for kernelversion 6.9.9

Board support for machines without known users

configname: CONFIG_UNUSED_BOARD_FILES

Linux Kernel Configuration
└─>Boot options
└─>Board support for machines without known users
In linux kernel since version 4.14.326 (release Date: 2023-09-23)  
Most ATAGS based board files are completely unused and are
scheduled for removal in early 2023, and left out of kernels
by default now. If you are using a board file that is marked
as unused, turn on this option to build support into the kernel.

To keep support for your individual board from being removed,
send a reply to the email discussion at
https://lore.kernel.org/all/CAK8P3a0Z9vGEQbVRBo84bSyPFM-LF+hs5w8ZA51g2Z+NsdtDQA@mail.gmail.com/

depends
CONFIG_ATAGS