No, they are not. The software can not make up for hardware limitations and the PS/2 spec. While the kernel does detect the keyboard and it "just works", there is no assurance that the hot-plug will not physically damage the motherboard.
Specifically, per the PS/2 spec, there is support for the current transients that can occur during hor plugging, or for the handling of potential static discharged. The first can blow a microfuse on the motherboard, the second can fry the keyboard interface chip or southbridge if it is integrated. Granted, most motherboards these days are more robust, but it is not a requirement. Try it on an actual (ancient!) IBM PS/2 or similar old computer enough times, and you might find you need a motherboard replaced (or surgery on the existing one).
Using your logic, regular PCI cards (not server level PCI-X or PCI-E) and IDE disk drives are also hot pluggable, in that, hey, if you do it quickly (and happen to have the card edge/connector angled just right so the ground lines connect first, etc) it sometimes works!