I couldn't find any official documentation specifying what each parameter to a `sla_miss_callback` callable are. This would be a great addition to know how to properly format the messages sent.
airflow/models/dag.py
if as_pk_tuple:
                condition = TI.filter_for_tis(TaskInstanceKey(*cols) for cols in tis.all())
                if condition is not None:
                    query = session.query(TI).filter(condition)
            if visited_external_tis is None:
                visited_external_tis = set()
            for ti in query.filter(TI.operator == ExternalTaskMarker.__name__):
                ti_key = ti.key.primary
                if ti_key in visited_external_tis:
                    continue
                visited_external_tis.add(ti_key)
                task: ExternalTaskMarker = cast(ExternalTaskMarker, copy.copy(self.get_task(ti.task_id)))
                ti.task = task
                if max_recursion_depth is None:
                    
                    
                    max_recursion_depth = task.recursion_depth
                if recursion_depth + 1 > max_recursion_depth:
                    
                    raise AirflowException(
                        "Maximum recursion depth {} reached for {} {}. "
                        "Attempted to clear too many tasks "
                        "or there may be a cyclic dependency.".format(
                            max_recursion_depth, ExternalTaskMarker.__name__, ti.task_id
                        )
                    )
                ti.render_templates()
                external_tis = (
                    session.query(TI)
                    .join(TI.dag_run)