The button for retreat is relocated to the second line, first entry of the grid due to the fact that reverse is in the top right. If you are using gridkeys you can retreat these units by pressing A or just pressing the button with your mouse. You can also press tab to cycle through your units and then use the normal hotkey to retreat infantry units.
Well but in my brain T or R or Z idk atm ( well not in my brain maybe in finger) But when u hear the Arty is incoming u dont think about mhhhhhhh T or A its just pressing so fast as u can the retread button! And in a game like CoH2 a good reaction should be an advantage...