The issue is relevant only to domain discovery in optaplanner-quarkus.

The following setup:

// with a public class, detection works correctly
abstract class AbstractIdentifiable {

    private Long id;

    @PlanningId
    public Long getId() {
        return id;
    }
}

public class TestdataEntity extends AbstractIdentifiable {
...
}

results in:
"java.lang.IllegalArgumentException: The class (class org.optaplanner.quarkus.testdata.superclass.domain.TestdataEntity) has 2 members ([public java.lang.Long org.optaplanner.quarkus.testdata.superclass.domain.TestdataEntity.getId(), public java.lang.Long org.optaplanner.quarkus.testdata.superclass.domain.AbstractIdentifiable.getId()]) with a PlanningId annotation."

suggesting there are two getters with the @PlanningId annotation, which is not the case.