I think OpenBSD (and other OSes) that do not allow v4 on v6 sockets might be on to something. It was felt convenient for application programmers to only have to listen to one socket for two protocols, but then code after needs to make very sure they know how to handle both families in the correct way for logging and so on, and now much later on it confuses someone that has to dig very deep into why the v4-box talks v4 without using v4.

Could be an indication that in the long run, just forcing applications that need both protocols to have separation so you are sure of what they are doing was the correct choice.