Repro steps:
1. Open attached project
2. In the Asset folder, double click on "ToonShading" Shader Graph element to open Shader Graph window
3. Find the "Custom Function" Node, that is attached to the rest of the graph, and click on the cogwheel
4. Change the Type field to "File"
5. Double click on the "CustomLighting" file, that appears in the Source field
6. Add "#pragma multi_compile _ _MAIN_LIGHT_SHADOWS" as the first line of the method and press "Save"
7. Go back to Shader Graph window, press "Save Asset" and observe the "Sphere" object in the Scene view

Expected result: Shadows are cast on the sphere
Actual result: No shadows are cast on the sphere

Reproducible with: 2019.2.0b6, 2019.2.1f1, 2019.3.0a12
Could not test with: 2017.4.31f1 (Universal RP did not exist), 2018.4.6f1, 2019.1.14f1, 2019.2.0b5 (project gets corrupted)

Reproducible with package versions: 6.9.1, 7.0.1
Could not test with package versions: 6.7.1 (project gets corrupted)

Note: Using Type "String", with the line from step 6 added to the front, casts shadows and works as intended