CONFIG_ARCH_MULTIPLATFORM is not available for thedefaultarchitecture x86.
Result is shown for architecture arm
default or selected kernelversion does not have config value CONFIG_ARCH_MULTIPLATFORM.
Result is shown for kernelversion 6.9-rc4

Require kernel to be portable to multiple machines

configname: CONFIG_ARCH_MULTIPLATFORM

Linux Kernel Configuration
└─>System Type
└─>Require kernel to be portable to multiple machines
In linux kernel since version 3.1 (release Date: 2011-10-24)  
In general, all Arm machines can be supported in a single
kernel image, covering either Armv4/v5 or Armv6/v7.

However, some configuration options require hardcoding machine
specific physical addresses or enable errata workarounds that may
break other machines.

Selecting N here allows using those options, including
DEBUG_UNCOMPRESS, XIP_KERNEL and ZBOOT_ROM. If unsure, say Y.