In https://github.com/elastic/elasticsearch/pull/85796 the `version.build_flavor` field was removed from the `/` API response. This API is marked as `stable` in the [API spec stub](https://github.com/elastic/elasticsearch/blob/master/rest-api-spec/src/main/resources/rest-api-spec/api/info.json) and thus this removal should be considered a regression. The removal has started [causing issues for users of clients](https://github.com/elastic/elasticsearch-java/issues/335) when they connect to 8.3.0+ of Elasticsearch.

My proposed fix is to add back the field with a static value of `default` and to change the removal of the field to the next major version instead.
server/src/main/java/org/elasticsearch/action/main/MainResponse.java
MainResponse other = (MainResponse) o;
        return Objects.equals(nodeName, other.nodeName)
            && Objects.equals(version, other.version)
            && Objects.equals(clusterUuid, other.clusterUuid)
            && Objects.equals(build, other.build)
            && Objects.equals(clusterName, other.clusterName);
    }
    @Override
    public int hashCode() {
        return Objects.hash(nodeName, version, clusterUuid, build, clusterName);
    }
    @Override
    public String toString() {
        return "MainResponse{"
            + "nodeName='"
            + nodeName
            + '\''
            + ", version="
            + version
            + ", clusterName="
            + clusterName
            + ", clusterUuid='"
            + clusterUuid
            + '\''
            + ", build="
            + build
            + '}';
    }
}