I checked the IC-7410 service manual, which you can find easily with Google, to see if I can connect the keyer output signal from ATmega32u4 used in an Arduino Leonard board directly to the ELE-KEY inputs of my rig. The input signals are named “DOTK” and “DASHK”, and if you trace the lines, you will find that the both signals go directly into the Front CPU (M30622F8PGP microcontroller), to which the Vcc of 3.3V is supplied.
Actually, the voltage at the ELE-KEY inputs is measured to be around 3.3V. Since the Vcc of ATmega32u4 on the Leonard board is 5V, we need to use either a level converter, most simply with two resistors, or an open collector (drain) circuit. It seems that the latter is the preferred choice, because it is a more generic approach.