They are extremely common actually. Why do you think the standard iPhones only does USB 2.0 transfer speed? The high-speed signal pins are simply not there, but the connectors themselves are still standard compliant.
Lower transfer rate means less shielding is needed for the cable as well as the overmold, and enables longer and more flexible cables, as extra shielding stiffens the cables.
> Why do you think the standard iPhones only does USB 2.0 transfer speed?
Because they saved die space in silicon? Same reason the MacBook Neo only has a single USB 3 and a USB 2. It seems that their A-series Pro silicon only has hardware for a single USB 3, and their non-Pro silicon doesn't even have it at all. I highly doubt they are sparing pins from the connector, the complexity of making a special port variant for that far surely outweighs any potential savings.
Lower transfer rate means less shielding is needed for the cable as well as the overmold, and enables longer and more flexible cables, as extra shielding stiffens the cables.