CONFIG_PHYS_RAM_BASE_FIXED is not available for thedefaultarchitecture x86.
Result is shown for architecture riscv

Explicitly specified physical RAM address

configname: CONFIG_PHYS_RAM_BASE_FIXED

Linux Kernel Configuration
└─>Boot options
└─>Explicitly specified physical RAM address
In linux kernel since version 5.2 (release Date: 2019-07-07)