When switching Discover to use the Fields API, _type field is no longer returned in the fields response. Moreover, when explicitly requesting a list of fields (which includes _type), an exception is thrown:
type: "unsupported_operation_exception", reason: "Cannot fetch values for internal field [_type]."

Our understanding was that this would be supported until 8.0.0. Removing it in a minor could be considered a breaking change for Discover.