default or selected kernelversion does not have config value CONFIG_CPU_IBPB_ENTRY.
Result is shown for kernelversion 6.9.9

Enable IBPB on kernel entry

configname: CONFIG_CPU_IBPB_ENTRY

Linux Kernel Configuration
└─>Mitigations for CPU vulnerabilities
└─>Enable IBPB on kernel entry
In linux kernel since version 4.14.326 (release Date: 2023-09-23)  
Compile the kernel with support for the retbleed=ibpb mitigation.