Apple ships a C++ compiler but you can't use C++ with Apple's platform SDKs. IF you want to use C++ you need to write Obj-C++ glue code to access most of the system frameworks, including UI code.
I would look at it differently, i.e, obj-c++ is a valid supported language environment, that makes interopt with C++ pleasant at least compared to other interopt choices. Developers may choose to target Obj-C++ even out of preference.
Metal shaders != Metal API. Metal API is Obj-C. Metal shaders are written in MSL (Metal Shading Language). C++ is still not involved, though MSL is admittedly based on C++.
"The Metal programming language is a C++14-based Specification with extensions and
restrictions. Refer to the C++14 Specification (also known as the ISO/IEC JTC1/SC22/WG21
N4431 Language Specification) for a detailed description of the language grammar.
This section and its subsections describe the modifications and restrictions to the C++14
language supported in Metal."
Also, the MSL and Objective-C compilers are built on top of LLVM, written in C++.
I don't know what point you're trying to make. MSL doesn't link with the rest of your program. No matter what language you use for your program, the shaders are written in MSL. For my program if I use Rust or C++ or Obj-C or Swift or a microscopic magnet and a very careful hand, I write my Metal shaders in MSL.