default or selected kernelversion does not have config value CONFIG_8723AU_BT_COEXIST.
Result is shown for kernelversion 6.4.14

Realtek RTL8723AU BlueTooth Coexistence

configname: CONFIG_8723AU_BT_COEXIST

Linux Kernel Configuration
└─>Device Drivers
└─>Staging drivers
└─>Realtek RTL8723AU BlueTooth Coexistence
In linux kernel since version 3.15 (release Date: 2014-06-08)  
This option enables icoexistence with BlueTooth communications for the r8723au driver.
Unless you know that this driver will never by used with BT, or the target system has
limited memory, "Y" should be selected.