default or selected kernelversion does not have config value CONFIG_NETKIT.
Result is shown for kernelversion 6.7-rc4

BPF-programmable network device

modulename: netkit.ko

configname: CONFIG_NETKIT

Linux Kernel Configuration
└─>Device Drivers
└─>Network device support
└─>BPF-programmable network device
In linux kernel since version 4.14.330 (release Date: 2023-11-20)  
The netkit device is a virtual networking device where BPF programs
can be attached to the device(s) transmission routine in order to
implement the driver's internal logic. The device can be configured
to operate in L3 or L2 mode. If unsure, say N.

source code: