Steps to reproduce:
1. Create new Unity project using Built-In 2D Template
2. In Hierarchy create Pixel Perfect Camera
3. Install Universal Rendering Pipeline package via Package Manager
4. Open Render Pipeline Converter window
5. Select U2D to URP Pixel Perfect Converter
6. Enable “U2D To URP Pixel Perfect Converter”
7. Press “Scan”
8. Press “Convert Assets”
9. Remove Universal Rendering Pipeline package

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Actual results: Warning is thrown in the Console window “The type Unity.2D.PixelPerfect.Editor UnityEditor.Rendering.Universal.U2DToURPPixelPerfectConverter is being serialized by [SerializeReference], but is missing the [Serializable] attribute. To suppress this warning add [Serializable] or use [MakeSerializable].”
Expected results: No warnings are thrown after removing the package, converter data is either properly marked serializable or cleaned up when URP is removed

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Reproducible with versions: 6000.4.0b9, 6000.5.0a7
Can’t test with versions: 6000.0.67f1, 6000.3.9f1 (different window design, “U2D To URP Pixel Perfect Converter” not available)

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Tested on (OS): macOS Silicon Tahoe 26.2 (M1)