Your final reserve fuel + diversion fuel (or flight plan fuel without an alternate) may be less than 1800 kgs on a NG, BUT you are then flying with a FUEL LOW caution and have to follow the QRH procedure for that. Look it up. It does restrict the way you fly the aircraft.
While I might get a FUEL LOW caution during a diversion, I always carry enough fuel so I land at my destination with at leat 1800 kgs during normal operations. That's airmanship. To me.