I asked about this feature in Beta3 and Snotling said it was a good idea but I notice it still hasn't made it's way into Beta4.
Auto De-select would happen on a unit/stack that runs out of movement on it's current path. Right now when you reach 0 or 1 moves left this currently happens (so road/water/open terrain paths work fine). However when moving on Hills, Swamp, Forest, Snow, Desert when you run out of movement on the current path the game doesn't deselect and takes 2-3X as long to figure out that you can't move any further on the path. So I end up clicking on the next unit manually and creating paths to places I don't want to go (which even with A* takes 2-3 seconds and then next turn I have to re-path again).
The rule/code would be simple.
If (move !=0 && move < move cost to next square in move path) deselect;
KGB