Cybersecurity - Memes

Or you just hard-wire it to USB killer only and charge your phone exclusively with wireless charging.

That feels really insightful.

Saw this post earlier and was cranking some brain cycles in kinda the same way you were. My brain settled on a switch for the USB since you should just need a 5 pole/2 throw switch (think I'm using those terms correctly?) to go from regular USB function to kill mode. I think for my own peace of mind, I'd want it to be 3 throw though (normal, completely unconnected, and kill). My brain then went to the battery, which I see as the real design constraint.

Then I got to thinking about building it into a phone case. The case would need to plug in to your phone's USB port, then have an additional external connection; it would be this connection that is switched into normal/nothing/kill mode. Cases can be pretty bulky, so tucking a battery into there would be easier and still maybe evade detection.

All that said, I think I like your idea better.

Edit: spelling

Edit 2: additional thought: if you went with the case, you'd want to have it really difficult to remove. Like, requiring undoing some screws, especially if you can get some torx or other niche screw head design (bonus points for mix and matched screw heads).
My thoughts on that are that even if the case is identified as having a false port on it, it would be so difficult to remove that the "adversary" just plugs their info stealer into it anyway.