default or selected kernelversion does not have config value CONFIG_ATH9K_TX99.
Result is shown for kernelversion 5.9.9

Atheros ath9k TX99 testing support

configname: CONFIG_ATH9K_TX99

Linux Kernel Configuration
└─> Device Drivers
└─> Network device support
└─> Wireless LAN
└─> Atheros ath9k TX99 testing support

Say N. This should only be enabled on systems undergoing
certification testing and evaluation in a controlled environment.
Enabling this will only enable TX99 support, all other modes of
operation will be disabled.

TX99 support enables Specific Absorption Rate (SAR) testing.
SAR is the unit of measurement for the amount of radio frequency(RF)
absorbed by the body when using a wireless device. The RF exposure
limits used are expressed in the terms of SAR, which is a measure
of the electric and magnetic field strength and power density for
transmitters operating at frequencies from 300 kHz to 100 GHz.
Regulatory bodies around the world require that wireless device
be evaluated to meet the RF exposure limits set forth in the
governmental SAR regulations.