If we're talking about D3DCompile2(), I don't think that does log levels? We always pass VKD3D_SHADER_LOG_INFO, at least. The application may decide to pass NULL for "messages_blob", of course, but we still retrieve the messages from vkd3d_shader_compile() in that case; we'd just have to output them.
You're right, it doesn't. I somehow had gotten it into my mind that there was a way to disable warnings, but no...