我发现了问题。似乎钥匙不应该包括 settings 前缀。

工作代码如下:

public CreateIndexResponse createIndex(@NotNull String indexName) throws IOException {
    CreateIndexRequest request = new CreateIndexRequest();
    request.index(indexName);
    Settings.Builder settingsBuilder =
            Settings.builder()
                    .put("analysis.filter.autocomplete_filter.type", "edge_ngram")
                    .put("analysis.filter.autocomplete_filter.min_gram", "1")
                    .put("analysis.filter.autocomplete_filter.max_gram", "10")
                    .put("analysis.analyzer.autocomplete.type", "custom")
                    .put("analysis.analyzer.autocomplete.tokenizer", "standard")
                    .putList("analysis.analyzer.autocomplete.filter", "lowercase", "autocomplete_filter");
    request.settings(settingsBuilder);
    return elasticSearchClient.indices().create(request, RequestOptions.DEFAULT);
}