I think you're referring to the Sky Couch, which isn't just an empty middle seat, but converting three economy class seats into a bed (just) big enough for two

I think you're referring to the Sky Couch, which isn't just an empty middle seat, but converting three economy class seats into a bed (just) big enough for two to snuggle up. https://www.airnewzealand.com.au/economy-skycouch https://www.airnewzealand.com.au/economy-skycouch But you're right, the average GDS can't cope with this.