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As a person authorizing a product that can potentially emit unhealthy or unsafe levels of RF,This used to be, can be, and should continue to be limited on a hardware level. You can't flash a phone to emit "unhealthy or unsafe levels of RF" if the circuit itself won't let such amounts of power to flow to the radio. Of course, someone could still mess with the hardware itself, but that doesn't scale and (in case of breaking RF limits) is illegal.
> or a parent of teenagers using social media,
This is interesting because, in a way, it's telling me that you the maker of the device are my parent. The argument sounds like, "to prevent your child from flashing your device, we'll prevent you from flashing your device". How about giving me the tools to limit my child's access, if I desire to do so, while not limiting my access?
> or a developer of medical applications intended for mobile phone...
This is a large topic, but to a first approximation, I don't believe a software that cannot be modified by the end-user for health reasons should be allowed on the phone. I also don't believe most medical apps in the smartphone spaces are of this category. The way I see it currently: either accept that some users (like myself) want to have full read & write access to their data on their phones, or make your solution a separate, tamper-proof, black-box device (and get a regulator to rule that for me to open it is illegal).