Well, I tried it but of course F1 only shows your *own* troops' morale, not the enemy's. So no way to test it that I can think of, unless there is a way to determine morale of enemy units.

The reference wasn't in the manual, Agent Miles - it's the window that appears in-game when you order an assassination on an enemy general. I can't remember the exact wording, but it tells you that, although a new general will be appointed, the assassination will have a "devastating" effect on the (enemy) army's morale for one year.

The more I think about it, I can't imagine how the mechanism for that would work in the game. Perhaps this is just one of those things they wanted to do but ultimately couldn't.