CONFIG_DEBUG_UART_FLOW_CONTROL is not available for thedefaultarchitecture x86.
Result is shown for architecture arm

Enable flow control (CTS) for the debug UART

configname: CONFIG_DEBUG_UART_FLOW_CONTROL

Linux Kernel Configuration
└─>Enable flow control (CTS) for the debug UART
In linux kernel since version 5.1 (release Date: 2019-05-05)  
Some UART ports are connected to terminals that will use modem
control signals to indicate whether they are ready to receive text.
In practice this means that the terminal is asserting the special
control signal CTS (Clear To Send). If your debug UART supports
this and your debug terminal will require it, enable this option.

depends
CONFIG_DEBUG_LL