This is what needs to be conformant: glTF™ 2.0 Specification.
Also this if you want to understand how transmission should work: glTF/extensions/2.0/Khronos/KHR_materials_transmission/README.md at main · KhronosGroup/glTF · GitHub
EDIT: Just to clarify why I pointed to two different spec links. Using alpha coverage for transmission (e.g., transparency effects like glass) is not the intention of the spec. This glass effect is the intention of the KHR_materials_transmission extension. That said, a lot of people misuse the alpha coverage of the core spec to represent glass. This is why we chose to default the experience to make this misused scenario work and have a flag to be conformant for other scenarios. The car model in the OP probably has been tweaked to look okay using alpha coverage, but it is not a physically accurate way to render glass.