I work in the AV industry. RS-232 is still the king for control signals between devices, even on brand new hardware that costs >10K USD. TV screens for signage/conference rooms often have RS-232 for more versatile control than HDMI-CEC. Higher bitrate than 9600 BPS is often not needed. The most common connector consists of three-pin screw terminals (Tx, Rx, GND), although these days most installations have at least one RS232-to-USB adaptor somewhere. And for larger rooms, RS232 is bridged over Ethernet.

This was a bit of a surprise when I started, but then I realised that many installations are decades old, with components having been replaced individually.