I think it has to do with the update cycle, or lack thereof. The iOS browser gets updated a couple times a year with new minor OS releases, whereas browsers for older OS versions get no updates ever. So if a bug is hardy enough to survive an entire year, it's there to stay.
Given Apple's opaque approach to support, even a good repro case of a bug in an upcoming iOS release is unlikely to ever make it to a fixed shape by the time said OS version is obsolete. So you have to switch for every single OS version, by name, since features are in fact in place, but work inconsistently between versions.
Before typing this, I just finished putting the first few new iOS9 switches into the app I'm developing, resurrecting various things that they killed in the beta. Maybe those switches can come out when they ship the real thing. But that has never happened before, so I'm not holding out any hope.