Once TaskGroups (https://airflow.readthedocs.io/en/latest/concepts.html#taskgroup) that would be released in Airflow 2.0 reach feature parity with SubDags and we have wide adoption and feedback from users about Taskgroups we should deprecate Subdags and remove them eventually in Airflow 3.0.

Discussion Thread: https://lists.apache.org/thread.html/ra52746f9c8274469d343b5f0251199de776e75ab75ded6830886fb6a%40%3Cdev.airflow.apache.org%3E
airflow/operators/subdag.py
self,
        *,
        subdag: DAG,
        session: Optional[Session] = None,
        conf: Optional[Dict] = None,
        propagate_skipped_state: Optional[SkippedStatePropagationOptions] = None,
        **kwargs,
    ) -> None:
        super().__init__(**kwargs)
        self.subdag = subdag
        self.conf = conf
        self.propagate_skipped_state = propagate_skipped_state
        self._validate_dag(kwargs)
        self._validate_pool(session)
    def _validate_dag(self, kwargs):