Unpack anywhere and make sure you open-xcom documents folder has the original game and mods you want to play with.
1. What is Sniper/Spotter AI, why is it a problem?
Basicly modders can set AI units to have either sniper or spotter role, or even both, and how long does the x-com soldier stays marked. But that's it, they can not change how the mechanic works, without changing the open-xcom engine. In "vanilla open-xcom" whenever a spotter spots an x-com unit, that x-com unit is marked as a target for snipers for a minimum of 1 turn(depending on modders). If x-com soldier breaks line of light or changes his position, he will still remain marked. In other words, if you don't have any cover or armor, you are as good as dead if you are marked. Remember the camouflage feature? Yeah it's totally useless if you hit an enemy spotter.
Quote: "It should be dangerous to deal with open fields and make you pause to consider whether to advance or retreat/abort." - ohartenstein23
2. Why is not this official and part of open-xcom release?
The author of the feature ohartenstein23 does not wish his Sniper/Spotter to be altered in any way.
"As I've said in other threads on the same topic, no. I am not going to neuter this feature to make it "more enjoyable", "more realistic", or "more logical". It fits within the logic of the current AI and how it behaves. I want to leave it as a tool for modders to provide challenge where previously there was just advantage to the player." - ohartenstein23
"The point of the spotter/sniper feature is not to make the AI play like a human player, but to offer the option to reduce one of the huge advantages the player has against them. Of course it's "cheaty" in some sense, there are many more "cheats" in the AI code to make them enough of a challenge.
So it's certainly possible to make that change, but as the author of the feature, I'm not going to blunt the challenge it adds just to make it more "realistic" or "fair"".- ohartenstein23
3. What exaclty was changed in the code?
P.S. Links to people complaining Sniper/Spotter Ai