Have a 3D mesh that takes advantage of Auto Smooth, then when exporting GLB, turn on Apply Modifier and see the Auto Smooth seems unchecked on all other mesh(s)! Surprised. I found out about this ...

Let's go into Three.js docs. They say that we can convert our normal GLTF files into Draco compressed GLTF files. In order to that we'd need to use something called glTF-Pipeline. So let's do this.