The wire is being used as an antenna. When you use UART, you have the other end of the wire terminated with another circuit. (i.e. The energy goes to into the connecting circuit.)

Because the wire ends at an open circuit, the currents on the wire radiate, turning the wire into an antenna.

Wires connected to a closed circuit will radiate too, if you drive them at high frequency. Maybe not efficiently, but it'll still radiate. There's nothing special about an open circuit that makes an antenna work.

Shielding (e.g. coax, or a properly grounded enclosure) is what prevents ordinary wires from becoming antennas.

Normal rs232 is run through shielded cables as well. It was standard practice to wire the d-sub connectors to the cable shields. The wires themselves may not have been shielded, but the cable was shielded against being an antenna.

Sometimes. Plastic DB9 to unshielded RJ12 is pretty common.

> When you use UART, you have the other end of the wire terminated with another circuit. (i.e. The energy goes to into the connecting circuit.)

The other end is likely a MOSFET with an input impedance on the order of 10 GΩ...