In which practical situation would a kill switch be useful and user-friendly? Not to mention production friendly. An antenna needs space at the sides of the phone. And a modular phone needs extra space as well. The FP4 is already way too big for my taste. Adding more buttons won’t make it better.
It’s a nice thought to be in control of your hardware. But in reality I don’t see the benefits. Are you protecting yourself against profiling for e.g. personalized ads? Your mic and camera are not used for that (yet). Are you worried about unsanctioned location requests? The moment you turn it on and navigate a few times to your home you lost that battle already. I can go on, also about WiFi and 4/5G connections. From what should this protect you against? Is it even happening? And once you turn it on to use it, isn’t it already futile to turn it off again?
To me this feature is a gimmick that comes at an unacceptable cost. I rather see an improved camera and microphone than a kill switch for it. And I think 99% would agree.
If you’re woried about your privacy, then don’t buy a smartphone. Switching off a part of that device because you’re not using it at that point won’t protect you from governments or ad companies. It will give a false sense of protection.
Switching off the front camera would be the only somewhat sensible feature. Less development cost and may be pushed for a good reason, but it’s still a false sense of privacy IMHO. If you can’t trust the software on your phone, then don’t use it.