default or selected kernelversion does not have config value CONFIG_ISDN_CAPI_CAPIFS_BOOL.
Result is shown for kernelversion 5.4.69

CAPI2.0 filesystem support (DEPRECATED)

configname: CONFIG_ISDN_CAPI_CAPIFS_BOOL

Linux Kernel Configuration
└─>Device Drivers
└─>ISDN support
└─>CAPI 2.0 subsystem
└─>CAPI2.0 filesystem support (DEPRECATED)
In linux kernel since version 2.6.20 (release Date: 2007-02-04)  
This option provides a special file system, similar to /dev/pts with
device nodes for the special ttys established by using the
middleware extension above.
You no longer need this, udev fully replaces it. This feature is
scheduled for removal.