In some places of our project we are using toMultimap() method which is marked as @Beta and consequently is considered as an unstable API.

See:
https://github.com/google/guava/blob/d7a26416109381a6770681bc6c299567615e8341/guava/src/com/google/common/collect/Multimaps.java#L108

So. There is already 29.0 version of Google Guava. When does it gonna be considered as a "stable API"?
I would provide you more examples of  similar "unstable API" methods from Guava...
public int size() {
      return fromMultimap.size();
    }
    @Override
    Collection<V2> createValues() {
      return Collections2.transform(
          fromMultimap.entries(), Maps.<K, V1, V2>asEntryToValueFunction(transformer));
    }
  }
  private static final class TransformedEntriesListMultimap<K, V1, V2>
      extends TransformedEntriesMultimap<K, V1, V2> implements ListMultimap<K, V2> {
    TransformedEntriesListMultimap(
        ListMultimap<K, V1> fromMultimap, EntryTransformer<? super K, ? super V1, V2> transformer) {
      super(fromMultimap, transformer);
    }
    @Override
    List<V2> transform(K key, Collection<V1> values) {
      return Lists.transform((List<V1>) values, Maps.asValueToValueFunction(transformer, key));
    }
    @Override
    public List<V2> get(K key) {
      return transform(key, fromMultimap.get(key));
    }
    @SuppressWarnings("unchecked")
    @Override
    public List<V2> removeAll(Object key) {
      return transform((K) key, fromMultimap.removeAll(key));
    }
    @Override
    public List<V2> replaceValues(K key, Iterable<? extends V2> values) {