CodexAG To some extent it depends on exactly what one is worried about.
If a phone allows physically turning off the microphone except when a voice call is in progress, that protects against audio being captured when a voice call is not in progress even if the software stack on the phone is exploitable.
But if the software stack is exploitable, then it could capture and exfiltrate in-call audio, and a hardware kill switch on the microphone would do nothing to stop that.
GrapheneOS is highly focused on avoiding exploits of the software stack -- through a combination of careful practice, software tools (e.g., hardened malloc), and Pixel hardware features (verified boot, hardware key management, MTE... many things). To whatever extent that works, hardware kill switches would be a "defense in depth" addition rather than foundational.
At present phones with hardware kill switches do not have strong hardware and software-stack security (or, for that matter, open firmware, something many people would view as a security and privacy win). It would be great to have everything in one device, but that is not the current situation.
Please note that I do not speak for the GrapheneOS project.