Bring back SHUTDOWN-related constants

This should restore code compatibility to 2.6 and prior. Note that the
constants would not appear in documentation since they are deprecated
and not use in Airflow core anyway anymore.

airflow/utils/state.py

@@ -58,6 +58,14 @@ class TaskInstanceState(str, Enum):
     SKIPPED = "skipped"  # Skipped by branching or some other mechanism
     DEFERRED = "deferred"  # Deferrable operator waiting on a trigger
 
+    # Not used anymore, kept for compatibility.
+    # TODO: Remove in Airflow 3.0.
+    SHUTDOWN = "shutdown"
+    """The task instance is being shut down.
+
+    :meta private:
+    """
+
     def __str__(self) -> str:
         return self.value
 
@@ -100,6 +108,14 @@ class State:
     SKIPPED = TaskInstanceState.SKIPPED
     DEFERRED = TaskInstanceState.DEFERRED
 
+    # Not used anymore, kept for compatibility.
+    # TODO: Remove in Airflow 3.0.
+    SHUTDOWN = TaskInstanceState.SHUTDOWN
+    """The task instance is being shut down.
+
+    :meta private:
+    """
+
     finished_dr_states: frozenset[DagRunState] = frozenset([DagRunState.SUCCESS, DagRunState.FAILED])
     unfinished_dr_states: frozenset[DagRunState] = frozenset([DagRunState.QUEUED, DagRunState.RUNNING])
 
@@ -190,6 +206,15 @@ def color_fg(cls, state):
     A list of states indicating that a task or dag is a success state.
     """
 
+    # Kept for compatibility. DO NOT USE.
+    # TODO: Remove in Airflow 3.0.
+    terminating_states = frozenset([TaskInstanceState.SHUTDOWN, TaskInstanceState.RESTARTING])
+    """
+    A list of states indicating that a task has been terminated.
+
+    :meta private:
+    """
+
     adoptable_states = frozenset(
         [TaskInstanceState.QUEUED, TaskInstanceState.RUNNING, TaskInstanceState.RESTARTING]
     )

tests/www/views/test_views_cluster_activity.py

@@ -115,6 +115,7 @@ def test_historical_metrics_data(admin_client, session, time_machine):
             "restarting": 0,
             "running": 0,
             "scheduled": 0,
+            "shutdown": 0,
             "skipped": 0,
             "success": 2,
             "up_for_reschedule": 0,
@@ -143,6 +144,7 @@ def test_historical_metrics_data_date_filters(admin_client, session):
             "restarting": 0,
             "running": 0,
             "scheduled": 0,
+            "shutdown": 0,
             "skipped": 0,
             "success": 0,
             "up_for_reschedule": 0,