After making Schema.Field#defaultValue package private, there is no way to distinguish whether a Field has a default value of 'null' or has no default at all. This is a breaking change, notably affecting Hortonworks Schema Registry. See discussion here: https://github.com/hortonworks/registry/pull/547
I propose we should make that method public and still deprecated. Otherwise I don't see a clear way to release a version of the Hortonworks registry that will be compatible with both Avro 1.8.x and 1.9.0.