Agreed, but that's an issue for all "interface types" on API boundaries (another similar issue is vector and matrix types in math libraries). IMHO a C string library must still be able to efficiently consume and produce vanilla zero-terminated "const char*" string data (and at a stable address, unlike for instance std::string) even if they mostly work with {pointer,length} ranges internally.

C strings are so ubiquitous that the ABI impedance mismatch is just too annoying.