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 static <T, K, V, M extends Multimap<K, V>> Collector<T, ?, M> flatteningToMultimap(
      java.util.function.Function<? super T, ? extends K> keyFunction,
      java.util.function.Function<? super T, ? extends Stream<? extends V>> valueFunction,
      java.util.function.Supplier<M> multimapSupplier) {
    checkNotNull(keyFunction);
    checkNotNull(valueFunction);
    checkNotNull(multimapSupplier);
    return Collector.of(
        multimapSupplier,
        (multimap, input) -> {
          K key = keyFunction.apply(input);
          Collection<V> valuesForKey = multimap.get(key);
          valueFunction.apply(input).forEachOrdered(valuesForKey::add);
        },
        (multimap1, multimap2) -> {
          multimap1.putAll(multimap2);
          return multimap1;
        });
  }
  /**
   * Creates a new {@code Multimap} backed by {@code map}, whose internal value collections are
   * generated by {@code factory}.
   *
   * <p><b>Warning: do not use</b> this method when the collections returned by {@code factory}
   * implement either {@link List} or {@code Set}! Use the more specific method {@link
   * #newListMultimap}, {@link #newSetMultimap} or {@link #newSortedSetMultimap} instead, to avoid
   * very surprising behavior from {@link Multimap#equals}.
   *
   * <p>The {@code factory}-generated and {@code map} classes determine the multimap iteration
   * order. They also specify the behavior of the {@code equals}, {@code hashCode}, and {@code