Even though the massive rule change delayed to 2022 there are still some changes to the technical regulations for 2021, with a new ban on floor slots and a triangular cutout from the maximum width at the rear face of the cockpit entry template, to 650mm from the car centreline at the rear axle line. The idea being to cut some downforce.
Nice illustration from F1.com - looks like Sauber provided a car.
There's also a 3kg increase in the minimum weight - on the power unit increasing from 147kg to 150kg. There is a change of definition making it explicit that 4 studs can be used for the gearbox mounting to the ICE, and a stipulation banning the DAS steering.