Deal with it, frankly. A cheap USB mic attached to the external screen, or for that matter some screens (including fairly high end ones like that LG 5k display

Deal with it, frankly. A cheap USB mic attached to the external screen, or for that matter some screens (including fairly high end ones like that LG 5k display Apple was featuring for a while) have built-in mics themselves these days. But this is one case where there really is a pretty fundamental trade off if the goal is a really simple core "when the lid is shut I'm guaranteed audio privacy from that vector". Anything that allows getting around that in turn is a potential bypass. Anything in software, even if it requires a special boot to access, leaves some room for bugs. And for both software or even a physical hardware switch there would be room for someone to accidentally or maliciously leave it on, and it would be very hard to notice. Given how cheap microphones are and that if someone is using the system with the lid closed they're already committing to a certain amount of much more expensive external hardware, I think it's quite reasonable to stick a straightforward visual heuristic that "if the system is closed and thus the camera cannot see the microphone cannot work either". Easy to verify at a glance for anyone, not just the owner, easy to remember. Like security, good privacy design requires not just a solid technical foundation, but considerations for human UX.