I'm actually working on one... well, it's a lot more than a C++ to C transpiler, but it's just one of the features it contains. It's actually a C/C++ derived language which currently aims to fully support C23 and C++17 in addition to its own extensions which bring in features from other languages like PHP, Rust, Python, Perl, Ruby, JS, etc, as well as auto-including headers, and auto-resolving namespaces. It can JIT execute and also generate executables, as well as emit standard C code you can compile with GCC or CLANG.
https://github.com/derekbsnider/madc