How to reproduce:
1. Create and open a new project
2. Open the Package Manager
3. In the Package Manager window click the “+” button and select “Install package by name”
4. In the “Name” field, type “com.unity.importer.usd” and click “Install“
5. Observe the Console window

Expected result: Warnings do not appear in the Console
Actual result: Warnings appear in the Console

Reproducible with: 1.0.0-pre.1 (6000.3.0a5, 6000.3.0b4, 6000.4.0a1), 1.0.0-pre.2 (6000.3.0a5, 6000.3.0b4, 6000.4.0a1)
Not reproducible with: 1.0.0-pre.1 (6000.0.58f2, 6000.3.0a4), 1.0.0-pre.2 (6000.0.58f2, 6000.2.6f2, 6000.3.0a4)

Reproducible on: macOS 15.3.2 (24D81) (M1 Max)
Not reproducible on: No other environments tested

Notes: The regression is related to Editor versions
Multiple “Type serialized via [SerializeReference] is missing the [Serializable] attribute” warnings are being thrown{color:#ffffff} {color}