Repro steps:
1. Use a project with Universal RP
2. Create a new material, assign to it shader "Legacy Shader/Diffuse"
3. Go to Edit -> Render Pipeline -> Universal Render Pipeline -> Upgrade Project Materials to Universal RP materials
4. Check console for errors

Expected result: no errors
Actual result: errors are generated

Note: this does not just happen for _Shininess. Other shaders will generate errors for other properties. For instance, "mobile/bumped specular" will generate an error for _Color.