S.T.A.L.K.E.R. 2: Heart of Chornobyl

To unlock this, you need to make a few key decisions towards the end of the game.

As far as I know, your decisions and alliances prior to Subtle Matter don't affect the ending, but do alter the missions and objectives that you have to complete on the way.

To unlock the Project Y achievement, you need to make the following decisions.

To target different endings, you'll need to make a manual save before each of these decision points. You can reload these to play the alternative paths after completion.

Subtle Matter (Burnt Forest):
In Orbita Station, after the "Get Into the Pod" objective, watch the cutscene and you will be confronted by Scar upon exiting the pod. When prompted, select the "Life is for the Living" option, then leave the complex.

Dangerous Liaisons (Malachite):
Just follow the objectives until you get to the Troposphere Comm Station. I had the ward pass, so used that to enter, rather than sneaking in. Keep going until you get the objective to "Activate the Control Panel". During the following cutscene, select the "Escape" option.

Now play the game as normal, completing the assigned main story missions and objectives until you reach The Last Wish in Chornobyl Anomalous Zone.

The Last Wish (Chornobyl Anomalous Zone):
During The Last Wish, you visit the doctor. In order to get the Project Y ending, you must lower your gun at the prompt.

Now keep going through the remaining main story missions until you reach the end of the game.

Project Y should unlock during the final cutscene, though in some cases you may need to wait for the credits to start rolling, then skip to the main menu with the button.