default or selected kernelversion does not have config value CONFIG_ATH9K_LEGACY_RATE_CONTROL.
Result is shown for kernelversion 5.9.9

Atheros ath9k rate control


Linux Kernel Configuration
└─> Device Drivers
└─> Network device support
└─> Wireless LAN
└─> Atheros Wireless Cards
└─> Atheros ath9k rate control

Say Y, if you want to use the ath9k specific rate control
module instead of minstrel_ht. Be warned that there are various
issues with the ath9k RC and minstrel is a more robust algorithm.
Note that even if this option is selected, "ath9k_rate_control"
has to be passed to mac80211 using the module parameter,