I don't know if this is what you meant, but plenty of game engines definitely have separate backends for Direct3D and OpenGL that are more than just translation

I don't know if this is what you meant, but plenty of game engines definitely have separate backends for Direct3D and OpenGL that are more than just translation shims. There's the big ones like Unreal, Unity, etc. The same is true of smaller in-house engines I've seen and worked on. Even open source hobbyist-driven engines like Godot have multiple renderer backends. It is true that there is a fair amount of translation going on (browsers and ANGLE is a big example, along with the recent MoltenVK) but it's by no means the only way people do things.