default or selected kernelversion does not have config value CONFIG_SPECULATION_MITIGATIONS.
Result is shown for kernelversion 6.9-rc5

Mitigations for speculative execution vulnerabilities

configname: CONFIG_SPECULATION_MITIGATIONS

Linux Kernel Configuration
└─>Mitigations for speculative execution vulnerabilities
In linux kernel since version 4.14.326 (release Date: 2023-09-23)  
Say Y here to enable options which enable mitigations for
speculative execution hardware vulnerabilities.

If you say N, all mitigations will be disabled. You really
should know what you are doing to say so.