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.
It is actualy legally considered stealing. There might be countries where their service centre TOS are allowed to overwrite common law, but that would be outliers. In virtually all countries it’s just theft.