CONFIG_ARM64_WORKAROUND_SPECULATIVE_AT is not available for thedefaultarchitecture x86.
Result is shown for architecture arm64

CONFIG_ARM64_WORKAROUND_SPECULATIVE_AT (not configurable)

configname: CONFIG_ARM64_WORKAROUND_SPECULATIVE_AT

Linux Kernel Configuration
└─>Kernel Features
└─>ARM errata workarounds via the alternatives framework
└─>ARM64_WORKAROUND_SPECULATIVE_AT
In linux kernel since version 5.1 (release Date: 2019-05-05)