If you want a module to be accepted in to the Linux kernel upstream, then you have to comply with the kernel's rules.
The kernel uses various C macros and GCC extensions. Ensuring that a module written in another language was compatible, even given that it would need to be built outside the normal tree would make its maintenance within the kernel tree impossible.
In terms of being written in C++/Rust/whatever, as an external module that complies with the kernel ABI (not guaranteed between kernel releases), go for it.
But if you want to have your protocol/module in the standard kernel tree, C is your choice.
The kernel uses various C macros and GCC extensions. Ensuring that a module written in another language was compatible, even given that it would need to be built outside the normal tree would make its maintenance within the kernel tree impossible.
In terms of being written in C++/Rust/whatever, as an external module that complies with the kernel ABI (not guaranteed between kernel releases), go for it.
But if you want to have your protocol/module in the standard kernel tree, C is your choice.