Not deleted from the project assets folder. The exact term would be "not referenced". When Unity processes a build, it only includes assets that are referenced in the scenes instead of just grabbing everything in the project. In my original case, I had more than 4GB of content referenced in a single scene, because I had one scene with a bunch of procedural systems that I implemented. What I did to get around the 4GB limit (of referenced assets) per scene was break my project into several scenes and move a few procedural systems into each scene. Then I used the multi-scene load option in Unity to load all of my scenes together at startup. My build is currently over 5GB total size, and each scene is less than 4GB. Unity still needs to properly address this bug. At a minimum, Unity should generate a pop-up to let the developer know they crossed the 4GB size limit on a scene in their build. But longer term, Unity should simply fix this stupid bug. This bug has wasted a lot of developer time.