### Description

ShutdownShardMigrationStatus serializes ShardAllocationDecision, which, as of #97803, is chunked. ShardAllocationDecision grows with the number of nodes. Therefore, it would save heap space to chunk ShutdownShardMigrationStatus as well.
x-pack/plugin/shutdown/src/main/java/org/elasticsearch/xpack/shutdown/RestGetShutdownStatusAction.java
public class RestGetShutdownStatusAction extends BaseRestHandler {
    @Override
    public String getName() {
        return "get_shutdown_status";
    }
    @Override
    public List<Route> routes() {
        return List.of(
            new Route(RestRequest.Method.GET, "/_nodes/{nodeId}/shutdown"),
            new Route(RestRequest.Method.GET, "/_nodes/shutdown")
        );
    }
    @Override
    protected RestChannelConsumer prepareRequest(RestRequest request, NodeClient client) {
        String[] nodeIds = Strings.commaDelimitedListToStringArray(request.param("nodeId"));
        return channel -> new RestCancellableNodeClient(client, request.getHttpChannel()).execute(
            GetShutdownStatusAction.INSTANCE,
            new GetShutdownStatusAction.Request(nodeIds),
            new RestToXContentListener<>(channel)
        );
    }
}