eg
```
"impacts": [
            {
              "severity": 3,
              "description": "Searches might return slower than usual. Fewer redundant copies of the data exist on 2 indices [.ds-logslogslogs-2022.03.29-000001, my-index-000001].",
              "impact_areas": [ "search" ]
            }
          ]
```
server/src/main/java/org/elasticsearch/cluster/routing/allocation/ShardsAvailabilityHealthIndicatorService.java
"initializing_replicas",
                        replicas.initializing,
                        "restarting_replicas",
                        replicas.unassigned_restarting,
                        "started_replicas",
                        replicas.started + replicas.relocating
                    )
                );
            } else {
                return HealthIndicatorDetails.EMPTY;
            }
        }
        public List<HealthIndicatorImpact> getImpacts() {
            final List<HealthIndicatorImpact> impacts = new ArrayList<>();
            if (primaries.indicesWithUnavailableShards.isEmpty() == false) {
                String impactDescription = String.format(
                    Locale.ROOT,
                    "Cannot add data to %d %s [%s]. Searches might return incomplete results.",
                    primaries.indicesWithUnavailableShards.size(),
                    primaries.indicesWithUnavailableShards.size() == 1 ? "index" : "indices",
                    getTruncatedIndicesString(primaries.indicesWithUnavailableShards, clusterMetadata)
                );
                impacts.add(new HealthIndicatorImpact(1, impactDescription));
            }
            /*
             * It is possible that we're working with an intermediate cluster state, and that for an index we have no primary but a replica
             * that is reported as unavailable. That replica is likely being promoted to primary. The only impact that matters at this
             * point is the one above, which has already been reported for this index.
             */
            Set<String> indicesWithUnavailableReplicasOnly = new HashSet<>(replicas.indicesWithUnavailableShards);