In its services and repair terms and conditions, Google says it will keep devices sent in for repair if they have a non-OEM part.
Update:
Google changes repair policy after criticism of third-party parts ban Google previously said it would not return mailed-in devices using “unauthorized” parts.
You say you’re in Germany; that is all the difference. The U.S. and its legal system, government and lawmakers have all been bought and paid for. Big businesses do what they want here.