Librem 5 phone has such kill switches.

It could do both, but it does say “or whatever”, not “and”.

How would you have a user-configurable switch that physically disconnects things? The mechanism for that sounds complex. I'm not a hardware person, but I imagine you'd need to route the traces for each possible component to the switch and then have like a dip switch panel to control which behaviors are controlled by the switch. Either that or a software-controlled equivalent to a dip switch panel that can only be configured in the bootloader, otherwise the software-controlled physical disconnect would be no safer than a software disconnect.

Im not a hardware person either, but ex the button physically turns off the canera, and software polls for camera power and can respond

Ah I think I understand what you're suggesting now. This hypothetical switch is both a physical and software disconnect. Some features like the camera would be physically disconnected by the switch and therefore would not be user-configurable but then some other features (for example, GPS) could additionally be software disconnected at the same time.

That seems like a neat idea, but IMO I wouldn't trust the software-controlled half of it, so I'd end up only using the non-configurable physical portion of it.

That makes sense but it's also just a guess. The quote above would be equally applicable to an entirely software option that is toggled with a physical switch rather than an option in a menu.

Since these things are almost certainly digital devices, just having a switch that cuts power to them could work.

As soon as it is sufficiently complex, it becomes a "trust me bro" switch.