A recent example, if that helps. See point#2 https://polycount.com/discussion/234499/mini-tutorial-baking-seamless-normal-maps-for-gltf One important thing to note: splitting UVs on hard edges is only really necessary if you’re baking normal maps.
"If more shells are costly on rendering wouldnt it be better to avoid it?" IMHO this is a case of taking an assumption (which isn't necessarily wrong of course) and running with it without really putting it in context. We've all seen polygon density affecting performance : importing a sculpt into a regular 3d program can…