Stealth has really never been a whole army encompassing thing. Sure, armies don't broadcast the coordinates of their central command, but the general location of army is rarely a secret. Afterall, the point of an army is to project influence over an area. And an army doesn't need to fight the enemy to project influence - suffices if the army is simply present and ready to fight. Sure, there are special stealth sections of the army, called reconnaissance. They infiltrate through the enemy front lines and try to see what goes on behind them. They usually work with in tandem with artillery. For example. the reconnaissance unit finds the enemy command center, relays the coordinates to artillery and artillery bombs those coordinates from a safe distance. Another important function of reconnaissance is to sit at important intersections and watch where and how the enemy moves. Does the enemy bring reinforcements which might indicate an impending attack or does the enemy move the reinforcements away towards another front, which means that its ripe for taking.
However, there are some political entities in our world which can be equated to "stealth empires". We know them as terrorists, drug syndicates or mafia. They usually operate on an existing sovereign territory, hence the need to be very secretive. Once they somehow attain sovereignty, they no longer need to be secretive - see the real life example ISIS.
If the steath was to be somehow implemented then it would create a new way to play the game, which means that AI must be taught how to play this game style and also how to play against it as a non-stealth empire and also how to play stealth vs stealth game. This would be a lot of work for a single gimmick.
According to what I wrote in how real life works, there would be two completely opposite ways to implement it.
- Everything is stealthed - In this implementation the fog of war would be everywhere. One would still be able to see monsters and their systems, but one would not be able to see enemy ships moving even in player own system. Fog of war would be revealed only on stars where the player has his ships present or there is a radar installation on one of the planets. Radar detection would be based on a total sum of ship hulls in a system. Idea being that the bigger the hull point sum in a system, the easier it is to detect. This would also be used to detect stuff at a distance. For example a radar can detect a minimum of 20 hull points in its own system, then the minimum detectable hull points by that radar in the star 50 parsecs away would be 100 hull points. That radar would be able to detect ship activity on the other side of the galaxy aswell, but then the minimum detectable hull point sum in a system would have to be like 30 000.
Then the stealth technologies would unlock modules which would reduce the ship's hull point contributions to the sum of hull points in system. For example a cruiser with 50 hull points and with Emission Reduction module would only contribute 35 hull points to the system hull points sum. Fleets would reveal the exact composition of enemy army only when both are in the same system. There could also be modules called Onboard Scanner, which would take up weapon slots. This would enable to extend a little detection range around a fleet when it is deep inside the enemy territory so that they would not be totally blind.
- Stealth units are implied to work behind the curtain - This kind of implementation implies that the empire's army comes with stealth regiments by default and those stealth units are doing their best without needing to be commanded. In other words, there are no such things as stealth units or the possibility of going undercover. Stealth units exist merely as modifiers attached to ships through technologies or doctrines. Modifiers like "Scouting squads - Can see 2 jumps into Fog of War", "Hit&Run Skirmishers - All enemy fleets within 1 jump range suffer 5 damage every turn", "Infiltrators - -10% to enemy defenses" and so on.