default or selected kernelversion does not have config value CONFIG_X25_ASY.
Result is shown for kernelversion 6.8.2

X.25 async driver

modulename: x25_asy.ko

configname: CONFIG_X25_ASY

Linux Kernel Configuration
└─>Device Drivers
└─>Network device support
└─>Wan interfaces support
└─>X.25 async driver
In linux kernel since version 2.6.12  
Send and receive X.25 frames over regular asynchronous serial
lines such as telephone lines equipped with ordinary modems.

Experts should note that this driver doesn't currently comply with
the asynchronous HDLS framing protocols in CCITT recommendation X.25.

To compile this driver as a module, choose M here: the
module will be called x25_asy.

If unsure, say N.

source code: