Currently, `TransportGetShutdownStatusAction` is executed in a transport thread, this action can do a substantial amount of work (O(# shards) in some cases) in the transport thread preventing pending transport tasks to make progress and potentially causing cluster instability. This action should be dispatched into a different thread pool to avoid such issues.
x-pack/plugin/shutdown/src/test/java/org/elasticsearch/xpack/shutdown/TransportGetShutdownStatusActionTests.java
IndexMetadata imd = generateIndexMetadata(index, 3, 0);
        IndexRoutingTable indexRoutingTable = IndexRoutingTable.builder(index)
            .addShard(TestShardRouting.newShardRouting(new ShardId(index, 0), LIVE_NODE_ID, false, ShardRoutingState.STARTED))
            .addShard(TestShardRouting.newShardRouting(new ShardId(index, 0), SHUTTING_DOWN_NODE_ID, true, ShardRoutingState.STARTED))
            .build();
        
        canAllocate.set((r, n, a) -> Decision.NO);
        
        canRemain.set((r, n, a) -> n.nodeId().equals(SHUTTING_DOWN_NODE_ID) ? Decision.NO : Decision.YES);
        RoutingTable.Builder routingTable = RoutingTable.builder();
        routingTable.add(indexRoutingTable);
        ClusterState state = createTestClusterState(routingTable.build(), List.of(imd), SingleNodeShutdownMetadata.Type.REMOVE);
        ShutdownShardMigrationStatus status = TransportGetShutdownStatusAction.shardMigrationStatus(
            state,
            SHUTTING_DOWN_NODE_ID,
            SingleNodeShutdownMetadata.Type.REMOVE,
            true,
            clusterInfoService,
            snapshotsInfoService,
            allocationService,
            allocationDeciders
        );
        assertShardMigration(
            status,
            SingleNodeShutdownMetadata.Status.COMPLETE,
            0,
            containsString("[1] shards cannot be moved away from this node but have at least one copy on another node in the cluster")
        );
    }
    public void testOnlyInitializingShardsRemaining() {