Visually speaking, the new keyboard is strikingly similar to the previous model. In the first image you can see the slightly taller option ⌥ symbol on the new model.
The new keycaps measure ever-so-slightly thinner (about 1.25 mm, vs. 1.50 mm on the 2017 model), allowing for easier removal with less risk of damage. We also note that the space bar's inner workings have been subtly redesigned.
And of course, there's the new silicone layer draped over the delicate butterfly mechanisms.