A composable index template with no `template` defined in the body is mistakingly always assumed to not be a time series template. Even if it refers to a component template that has the `index.mode` setting set to `time_series` and the component template defines mappings with dimension fields or routing paths.

Incorrectly this causes to create a data stream with a tsdb index *without* time series start and end time settings set. The fact that this can happen is another bug and is being tracked #98833.

What should happen in this case is that a tsdb data stream is created with a valid tsdb backing index (with correctly generated time series start and end time settings).

This bug was found as part investigating #98792 and is the cause of that bug.
server/src/main/java/org/elasticsearch/cluster/metadata/Metadata.java
return aliasMd.indexRouting();
        }
        return routing;
    }
    private static void rejectSingleIndexOperation(String aliasOrIndex, IndexAbstraction result) {
        String[] indexNames = new String[result.getIndices().size()];
        int i = 0;
        for (Index indexName : result.getIndices()) {
            indexNames[i++] = indexName.getName();
        }
        throw new IllegalArgumentException(
            "Alias ["
                + aliasOrIndex
                + "] has more than one index associated with it ["
                + Arrays.toString(indexNames)
                + "], can't execute a single index op"
        );
    }
    /**
     * Checks whether an index exists (as of this {@link Metadata} with the given name. Does not check aliases or data streams.
     * @param index An index name that may or may not exist in the cluster.
     * @return {@code true} if a concrete index with that name exists, {@code false} otherwise.
     */
    public boolean hasIndex(String index) {
        return indices.containsKey(index);
    }
    /**
     * Checks whether an index exists. Similar to {@link Metadata#hasIndex(String)}, but ensures that the index has the same UUID as
     * the given {@link Index}.
     * @param index An {@link Index} object that may or may not exist in the cluster.