CONFIG_KEXEC_SIG is not available for the selected architecture x86.
Result is shown for architecture s390

Verify kernel signature during kexec_file_load() syscall

configname: CONFIG_KEXEC_SIG

Linux Kernel Configuration
└─>Processor type and features
└─>Verify kernel signature during kexec_file_load() syscall
In linux kernel since version 5.1 (release Date: 2019-05-05)
This option makes kernel signature verification mandatory for
the kexec_file_load() syscall.

In addition to that option, you need to enable signature
verification for the corresponding kernel image type being
loaded in order for this to work.
depends
CONFIG_KEXEC_FILE
CONFIG_MODULE_SIG_FORMAT