Servers have had memory mirroring as a feature for years. This seems like a cool extension of that technology. It would be an advantage in some systems where scaling out isn't an option and single node availability needs to be as high as possible.

Apparently, there's some coordination mechanism, where you tell the OS that you want to remove a certain memory stick, so it moves all the memory onto other RAM sticks (or uses paging to move it to your hard drive). Only then would you actually physically unplug the memory stick.

See, for example: https://docs.kernel.org/admin-guide/mm/memory-hotplug.html
(Mind that this is kernel documentation. If you actually want to do this, there's probably some CLI program to make it easier.)

USB devices are also hotpluggable, but that doesn't mean that the data stays in the system if you just pull out the HDD.