### 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.
server/src/main/java/org/elasticsearch/cluster/metadata/ShutdownShardMigrationStatus.java
this.allocationDecision = null;
        }
    }
    public long getShardsRemaining() {
        return shardsRemaining;
    }
    public String getExplanation() {
        return explanation;
    }
    public SingleNodeShutdownMetadata.Status getStatus() {
        return status;
    }
    @Override
    public XContentBuilder toXContent(XContentBuilder builder, Params params) throws IOException {
        builder.startObject();
        builder.field("status", status);
        builder.field("shard_migrations_remaining", shardsRemaining);
        if (Objects.nonNull(explanation)) {
            builder.field("explanation", explanation);
        }
        if (Objects.nonNull(allocationDecision)) {
            builder.startObject(NODE_ALLOCATION_DECISION_KEY);
            {
                
                ChunkedToXContent.wrapAsToXContent(allocationDecision).toXContent(builder, params);
            }
            builder.endObject();
        }
        builder.endObject();
        return builder;