Here is a photo of how the tiny hardware switches of the PinePhone look like:
View attachment 35795
In my opinions this is more a marketing tool to differentiate a brick with display from another brick with display.
The practical use is IMHO limited since with such tiny switches you tend to never use them. To operate such a device as a phone, you have to turn them all on for your phone call and then turn off again. Who will be regularily doing this? If they are never turned on, well one could buy a cheaper devices that does not even have the switched off hardware...
The other aspect is that some malicious software may sit there and wait until you turn them voluntarily on and then tap some data. So if it is able to tap any data, the hardware kill switch won't help.
Something else to consider is, since these devices (like the Pyra) are open source software devices, everyone will have a different system installed. And you may even have modified your kernel or user-space.
This makes it quite difficult for spyware to focus a well defined target installation. And with just some thousands of these devices it is also not interesting. Unless you are the target of a spying organization that invests a lot of energy to find you and analyse your specific device.
Regarding the software controlled "kill switches" of the Pyra, we can turn off the Modem power through a gpio controlled power FET. Well some AT command does the same, but just to be sure... You can check if this gpio is active from user-space, i.e. you can monitor if some malicious process is tampering with this gpio. For the microphone it is connected to the Audio ADC in the twl6040 chip. This can be powered off separately, or you can remove the code that handles any microphone from the kernel. For WiFi and Bluetooth we use a TiWi module which also has two power on/off gpios. Well, we do not know if they really turn off power, but I think TI could not sell these modules if the power-off function would be broken. So unless someone manages to replace your self-built kernel image, the software controlled hardware switches will stay off.
And finally about securing the data you transfer over the modem or WiFi which is sometimes discussed a the reason why people ask for power switches. The help is end-to-end encryption. It has been designed for unreliable and not trustworthy networks. And the modem is to be considered as part of the not trustworthy network. Yes, there are attacks to not well written WiFi firmware but Linux developers take care of this in the newest kernel releases. Which is a good reason to update the kernel from time to time.