I think sleeping on the back really has to do with making the face more physically attractive. There is no pressure put on your face by a pillow or the mattress so it can keep it’s proportional form. I sleep on my side and thus my cheekbones are flattened.
Sleeping on the side is just more comfortable and pleasurable. It’s too bad that it most likely effects the face from being full and symmetrical.
Sleeping on your side wouldn’t affect your cheekbones a bit, though I suppose it might, over time, have some sort of stretching effect on the overlying skin.
I would suggest that, unless you have filmed yourself sleeping for eight hours solid, you don’t really know what percentage of time you spend sleeping on your side.
I’ve a theory from watching puppies sleep together. If they lay on their side and another puppy lays their head on the first puppies head then symmetry is achieved. The proof is in the goofy response to puppies.
Lay on your side and put something that would equal the weight placed on your upper cheekbone and you too can have movie star cheekbones.
As for cheekbones, bones grow as a result of impact, so I suggest smacking yourself in the face to promote growth. You’re probably too old for it to work, but it’s work a go.
Like the old joke, “I want to die peacefully in my sleep like my granddad, not screaming like the passengers in his car”.
Babies, who have soft skulls, can have some deformation as a result of sleep position. It’s common for babies who are put to sleep on their backs (as is recommended) to develop a flat spot on the back of their head. But it fades away as their head grows.