Yes, most language features past the C89 subset are not supported, besides the C standard library, because C++ has much better alternatives, like why _Generic when templates are a much saner approach, than type dispatching with the pre-processor.

However that is besides the point, 99% of C89 code minus a few differences, is valid C++ code, and if the situation so requires, C++ code can be exactly the same way.

And lets not forget most FOSS projects have never moved beyond C89/C99 anyway, so stuff like _Generic is of relative importance.