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...
final Multimap<K, V1> fromMultimap;
    final EntryTransformer<? super K, ? super V1, V2> transformer;
    TransformedEntriesMultimap(
        Multimap<K, V1> fromMultimap,
        final EntryTransformer<? super K, ? super V1, V2> transformer) {
      this.fromMultimap = checkNotNull(fromMultimap);
      this.transformer = checkNotNull(transformer);
    }
    Collection<V2> transform(K key, Collection<V1> values) {
      Function<? super V1, V2> function = Maps.asValueToValueFunction(transformer, key);
      if (values instanceof List) {
        return Lists.transform((List<V1>) values, function);
      } else {
        return Collections2.transform(values, function);
      }
    }
    @Override
    Map<K, Collection<V2>> createAsMap() {
      return Maps.transformEntries(
          fromMultimap.asMap(),
          new EntryTransformer<K, Collection<V1>, Collection<V2>>() {
            @Override
            public Collection<V2> transformEntry(K key, Collection<V1> value) {
              return transform(key, value);
            }
          });
    }