2019 was a transition year for GM where they produced a "New Body Style" for the Chevrolet Silverado/GMC Sierra 1500. However, they continued the "Classic" through 2019 as well. There are two key differences between the two body styles shown below.
The wheel wells on the classic style are more box like where the new body style has a more circular wheel well.
The headlights on the classic body style have a large rectangle shape to them where the new body style headlights are more of a narrow rectangle shape to them.