Many of the Godot shader tutorials currently online are for Godot 2, but can easily be adapted to Godot 3.
For example, this smoke shader tutorial can be adapted to Godot 3 simply by adding a line at the top of the shader code which specifies which type of shader is being created.
Many of the online Godot 2 shader tutorials are like this. In Godot 2.0, you did not have to specify the shader type in the code, but for Godot 3 you do.
Like you, I work primarily on 2D games and am a novice when it comes to shaders. But I think they are definitely worth learning.
2D games like Hollow Knight, Salt & Sanctuary, and Slain! all make heavy use of shaders for their effects.