Steps to reproduce:

Create a new 3D URP project
In the Project window, create folder with a lot of characters (e.g. "TestShaderGraphMaxLength")

Inside of the folder, create another folder and continue doing that until the path is long:

Assets/TestShaderGraphMaxLength/TestShaderGraphMaxLength/TestShaderGraphMaxLength/TestShaderGraphMaxLength/TestShaderGraphMaxLength/TestShaderGraphMaxLength/TestShaderGraphMaxLength

Navigate to the deepest folder
Go to Assets > Create > Shader Graph > URP > Lit Shader Graph
Name it "TestShaderGraphMaxCharacterLenght.shadergraph"
Double click on the .shadergraph to open it
Observe the opened window and Console

Actual results: "Exception thrown while invoking [OnOpenAssetAttribute] method 'UnityEditor.ShaderGraph.ShaderGraphImporterEditor:OnOpenAsset (UnityEngine.EntityId,int)' : DirectoryNotFoundException: Could not find a part of the path" error is thrown and fully gray window called "MaterialGraphEditWindow" opens up

Expected results: No errors and the selected shader graph opens. Or the user is warned about using long paths in the Editor

Reproducible with versions: 6000.0.0f1, 6000.3.16f1, 6000.4.8f1, 6000.5.0b9, 6000.6.0a5

Tested on (OS): Windows 11

Notes:

Does not seem to reproduce on macOS
Does not reproduce when the .shadergraph asset is placed directly under Assets or under a shorter path
If the Shader Graph name is increased further, Unity may fail earlier and show a "Moving file failed" popup instead - The popup is also not very informative for long path cases, as it does not clearly explain that the failure is related to path length or provide actionable guidance