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

Allow configurations that result in non-portable kernels

configname: CONFIG_NONPORTABLE

Linux Kernel Configuration
└─>Platform type
└─>Allow configurations that result in non-portable kernels
In linux kernel since version 4.14.326 (release Date: 2023-09-23)  
RISC-V kernel binaries are compatible between all known systems
whenever possible, but there are some use cases that can only be
satisfied by configurations that result in kernel binaries that are
not portable between systems.

Selecting N does not guarantee kernels will be portable to all known
systems. Selecting any of the options guarded by NONPORTABLE will
result in kernel binaries that are unlikely to be portable between
systems.

If unsure, say N.

is selected by
CONFIG_ARCH_RV32I