It is possible to use without flow control.
However, if communication is performed without using flow control, there is a possibility that data will be lost, so there is concern that the reliability of module operation and communication may be adversely affected. We strongly recommend using flow control for reliable control and communication.

• Excerpt from BT121datasheet

For reliable communications data transfer the hardware flow control RTS/CTS signals must be used in the UART interface.

• Excerpt from the BGM111 datasheet

If UART is used as BGAPI serial protocol hostinterface it is strongly recommended to use RTS and CTS signals for reliable data transmission.

If you use Silicon Labs' EFM32 as a control microcontroller, please refer to another topic (EFM32: I want to perform hardware flow control with UART. How can I achieve this?).