Not on iOS anymore, that value is scrambled on a regular basis.

This is super cool. Was there any announcement or documentation for this?

We used a Cisco Meraki router once for a client and rigged it up to know who was in the office (for fun, to be aware that it could be done). It'd be nice to know the iPhone/iPad scramble themselves if possible.

Apple made this change in 2014, it was widely reported. [1] Apparently it exists on Android now also, though I don't follow that platform closely.

1. https://arstechnica.com/apple/2014/06/ios8-to-stymie-tracker...

Android does the same thing, both announced a while ago

Only if your phone is locked and it is looking for all open wifi networks. If you unlock it or it is connected to a particular wifi network this is not true.

No necessarily. If you are connected to some wifi and sending/receiving data, your MAC is still visible in the air.