On my phone, the modem can be killed with a hardware kill switch.

Which one? I have a Pinephone and it is a great device. It's just not a good phone.

Librem 5.

The if here is some phones can be turned off and not transmit anything, at all.

Modems are usually directly connected to the battery rail, because (so the saying goes) they have high peak power consumption.

I once built a cell phone from scratch. They do have high peak power consumption, and I did connect it to the battery rail directly, along with some capacitors to handle the surges.

I have a Pinephone, which has hardware switches to turn off potential privacy destroying features.

But it's even more secure than that - them made it so it's such a poor phone that I never managed to get it into use as a daily driver phone for more than a day or two at a time! The most private device is one you never bother carrying!

If you need to worry about cellular tracking, you should either carry your phone in a faraday bag or carry a dumb phone whose battery can be taken out.