This article gives me an excuse to mention my favorite obscure old keyboard switch type: the acoustic keyboard switch.

Back in the 80s when electronics were expensive, someone figured out that you could detect all key-presses on a keyboard with just a couple of microphones. There was no wiring to any of the key switches, instead there was a long metal bar underneath the keyboard with a microphone on each side. Pressing a key made a little metal hammer hit that bar, and the difference of arrival between the two microphones was then used to figure out which key had been pressed.

Here's a presentation from Keycon 2014 about it (starts at 1:46:01, description of how it works starts at 1:50:30): https://www.youtube.com/live/AvszDsr1js8?t=6361