default or selected kernelversion does not have config value CONFIG_ISDN_CAPI_CAPI20.
Result is shown for kernelversion 6.9.9

CAPI2.0 /dev/capi20 support

modulename: capi.ko

configname: CONFIG_ISDN_CAPI_CAPI20

Linux Kernel Configuration
└─>Device Drivers
└─>ISDN support
└─>CAPI 2.0 subsystem
└─>CAPI2.0 /dev/capi20 support
In linux kernel since version 2.6.20 (release Date: 2007-02-04)  
This option will provide the CAPI 2.0 interface to userspace
applications via /dev/capi20. Applications should use the
standardized libcapi20 to access this functionality. You should say
Y/M here.

source code: