As I noted in the feature request, some cables want both DTR and RTS on, and use ground for the voltage reference base. Others want RTS low and DTR high, and some want the reverse.
From my experience, only larger CI-V (, etc) converter boxes are dependent on RTS being high to power the circuit. I've seen no reports of issues with any cable on any typical VHF/UHF type setup, so I don't believe this to be a problem for those types of radios (which, of course, are the major target market for CHIRP). Further, I believe it is the overwhelming common practice (rightly so, IMHO) for a opened serial port to assert DTR unless configured to do otherwise. I believe that is likely the reason why we've seen no reports of this being a problem until now. At least on all my systems, CHIRP opening the port asserts DTR, which powers my MAX232-based custom interface cables.
I'm hesitant to expose such a setting in the UI without any indication of need or issues in the field because it becomes one more thing that can be set incorrectly by the user and thus one more thing that we must clarify, explain, etc in order to provide support.
The other important thing to note, of course, is that probably more than 90% of CHIRP's users are utilizing cables with inbuilt USB interfaces, which are always powered from the bus and for which DTR and RTS settings (for the purposes of powering a converter) are meaningless. We have no reasonable way of determining if a device selected by the user fits into this classification, which means exposing them in the UI provides only confusion, IMHO.
What commercially-available cables require RTS low and DTR high?
Since I'm quite certain this is only a problem with HF CI-V-like interface boxes, if anything, I'd like to restrict such a setting (if we were to add it) to those drivers that are likely to have such a requirement.
I'm not really sure what my preferred path forward would be. Perhaps a documented, but low-visibility config file option that would let someone enable it for a specific need? Again, with no reports of issues with this, I'm hesitant to expose it widely.