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
private interface TestDecider {
        Decision test(ShardRouting shardRouting, RoutingNode node, RoutingAllocation allocation);
    }
    @Before
    private void setup() {
        canAllocate.set((r, n, a) -> { throw new UnsupportedOperationException("canAllocate not initiated in this test"); });
        canRemain.set((r, n, a) -> { throw new UnsupportedOperationException("canRemain not initiated in this test"); });
        clusterInfoService = EmptyClusterInfoService.INSTANCE;
        allocationDeciders = new AllocationDeciders(
            List.of(new NodeShutdownAllocationDecider(), new NodeReplacementAllocationDecider(), new AllocationDecider() {
                @Override
                public Decision canAllocate(ShardRouting shardRouting, RoutingNode node, RoutingAllocation allocation) {
                    return canAllocate.get().test(shardRouting, node, allocation);
                }
                @Override
                public Decision canRebalance(ShardRouting shardRouting, RoutingAllocation allocation) {
                    
                    return Decision.NO;
                }
                @Override
                public Decision canRemain(
                    IndexMetadata indexMetadata,
                    ShardRouting shardRouting,
                    RoutingNode node,
                    RoutingAllocation allocation
                ) {
                    return canRemain.get().test(shardRouting, node, allocation);