Was the center viewport on selected unit quick key ever implemented? If so which quick key do you use?
Yes it was (sort of). The regular center-on-selected-unit key (C) checks that its inside the 'centering border' of the viewport. This is one of the BUG option values, that defaults to 2 I think. The effect is to re-center the viewport on the selected unit if its within that distance from the current viewport edge. It ONLY has any effect when you press the center-on-selection key, so just set that value to a larger number if you want that to re-center the viewport also if the unit is a bit further away from the edge.
I can easily also add a force-center if you hold down some modifier key while pressing the center-on-selection key though if you need that as well (assuming ctrl-C, shift-C, alt-C are not all taken already!)