Today submitting an unbatched `ClusterStateUpdateTask` to the `MasterService` is deprecated, and devs are encouraged to write their own batching executor instead. In principle this makes it possible to combine updates without incurring the costs of building a whole new `ClusterState` on each iteration, but in practice most implementations don't work at a lower level and just iteratively update a `ClusterState`. Moreover creating new `ClusterState` instances is nothing like as expensive as once it was.

We should accept this and provide some friendlier utilities for writing batched cluster state updates, ideally leaving the developer to express the desired per-task update as a `ClusterState -> ClusterState` operator.
server/src/main/java/org/elasticsearch/cluster/metadata/MetadataIndexTemplateService.java
+ " from existing composable templates [%s] with patterns (%s); this template [%s] may be ignored in favor"
                        + " of a composable template at index creation time",
                    request.name,
                    request.indexPatterns,
                    Strings.collectionToCommaDelimitedString(overlaps.keySet()),
                    overlaps.entrySet().stream().map(e -> e.getKey() + " => " + e.getValue()).collect(Collectors.joining(",")),
                    request.name
                );
                logger.warn(warning);
                HeaderWarning.addWarning(warning);
            } else {
                
                String error = String.format(
                    Locale.ROOT,
                    "legacy template [%s] has index patterns %s matching patterns"
                        + " from existing composable templates [%s] with patterns (%s), use composable templates"
                        + " (/_index_template) instead",
                    request.name,
                    request.indexPatterns,
                    Strings.collectionToCommaDelimitedString(overlaps.keySet()),
                    overlaps.entrySet().stream().map(e -> e.getKey() + " => " + e.getValue()).collect(Collectors.joining(","))
                );
                logger.error(error);
                throw new IllegalArgumentException(error);
            }
        }
        templateBuilder.order(request.order);
        templateBuilder.version(request.version);
        templateBuilder.patterns(request.indexPatterns);
        templateBuilder.settings(request.settings);