Version: 1.42.0-beta+linux64 but probably even older one.

I've set machine_max_acceleration_retracting as 1500mm/s and machine_max_acceleration_printing as 500mm/s but the generated gcode only add a single M204 P500 R1500 T500 ; sets acceleration (P, T) and retract acceleration (R), mm/sec^2

There is a little bit confusion here. The tooltip says that machine_max_acceleration_retracting is M204 T but M204 T is the travel acceleration, the retract acceleration is M204 R

So, how can I set a travel acceleration different from the retract acceleration ?
Slic3r seems to wrongly use the same value for both, regardless the tooltip.

I think a field is missing:

machine_max_acceleration_extruding is correct as M204 P
machine_max_acceleration_retracting tooltip is wrong. The proper gcode command is M204 R but M204 T is shown in tooltip
machine_max_acceleration_traveling is missing. The proper gcode command is M204 T but currently is exported with the same value as machine_max_acceleration_extruding

http://marlinfw.org/docs/gcode/M204.html