I've never seen such a rule. In all the games I've played, communication was allowed during retreat and build phases. Diplomacy would be a very different game without it.

Having communication during moves but not during retreat and builds would be very weird. It would mean that you had to plan for all build contingencies with potential allies in the Fall. Without communication, you and your ally will have trouble coordinating builds, and if you build the wrong thing, you could accidentally stab your ally (by building a unit that would be most useful against him, instead of against someone else).