Yes, developers will need to implement workaround in the case when A1 and B1 cannot use the same version of C1. So how does the function versions approach solve

Yes, developers will need to implement workaround in the case when A1 and B1 cannot use the same version of C1. So how does the function versions approach solve the problem? For using CI/CD to build every version of every app against every version of every dependencies, even if we just ignore the problem of combinatorial explosion, some software may be abandonware and some may be entirely new. The set of versions of dependencies that work with them may not even overlap, so the 'correct' combination may not even exist. I think the function versioning approach is just a way to maintain backward compatibility, it is not required for backward compatibility, nor enough for backward compatibility.