Subsurface scattering was proposed as an extension for GLTF back in 2018, but didn't make it into GLTF 2.0. Any idea why? Subsurface scattering is needed to get skin out of the uncanny valley.

I believe that KHR_materials_volume extension is what you're looking for.

https://www.khronos.org/blog/using-the-new-gltf-extensions-volume-index-of-refraction-and-specular​

https://gltf-transform.donmccurdy.com/classes/extensions.materialsvolume.html​

check the new extension KHR_materials_volume with KHR_materials_transmission