Not a plenum, just a continuation of room air space. No duct work or air flow under the floor tiles, just dead air space.
And yes, there is a probability of damaged cords. I've seen some of the floor openings with 3" plastic 'grommets', some without. And I've encountered several heavy boxes of paper jammed up on the cords. Looking under the tiles, I could see many cord ends semi-plugged in to the receptacles. They have about 5 duplex receptacles per 20 Amp circuit, and every available receptacle is used..so we have 10 plug strip cords running off of each 20 Amp circuit. These feed computer work stations, printers, and sometimes people think they need to put in a little space heater on the same circuit!