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

Use bootloader kernel arguments if available

configname: CONFIG_CMDLINE_FALLBACK

Built-in command line usage
└─>Use bootloader kernel arguments if available
In linux kernel since version 5 (release Date: 2019-03-03)  
Use the built-in command line as fallback in case we get nothing
during boot. This is the default behaviour.