There are several C API projects on HaikuArchives which are completely abandoned and have indeed bitrotted due to the API changes (libcharlemagne, Habid).
It is entirely possible to create a C API for the Haiku API, in the strictest sense (perhaps an extern_c generator + wrapper on the development headers) but as always with 3rd party bindings, there is a degree of risk of it bit-rotting.
Making it a first-party one included in the Haiku source might address this problem if it is extremely useful and it should have little to no dependencies included. Again this is up to the core devs to decide.