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...
* <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
   * toString} methods for the multimap and its returned views. However, the multimap's {@code get}
   * method returns instances of a different class than {@code factory.get()} does.
   *
   * <p>The multimap is serializable if {@code map}, {@code factory}, the sets generated by {@code
   * factory}, and the multimap contents are all serializable.
   *
   * <p>The multimap is not threadsafe when any concurrent operations update the multimap, even if
   * {@code map} and the instances generated by {@code factory} are. Concurrent read operations will
   * work correctly. To allow concurrent update operations, wrap the multimap with a call to {@link
   * #synchronizedSortedSetMultimap}.
   *
   * <p>Call this method only when the simpler methods {@link TreeMultimap#create()} and {@link
   * TreeMultimap#create(Comparator, Comparator)} won't suffice.
   *
   * <p>Note: the multimap assumes complete ownership over of {@code map} and the sets returned by
   * {@code factory}. Those objects should not be manually updated and they should not use soft,
   * weak, or phantom references.
   *
   * @param map place to store the mapping from each key to its corresponding values
   * @param factory supplier of new, empty sorted sets that will each hold all values for a given
   *     key
   * @throws IllegalArgumentException if {@code map} is not empty
   */
  public static <K, V> SortedSetMultimap<K, V> newSortedSetMultimap(
      Map<K, Collection<V>> map, final Supplier<? extends SortedSet<V>> factory) {
    return new CustomSortedSetMultimap<>(map, factory);
  }
  private static class CustomSortedSetMultimap<K, V> extends AbstractSortedSetMultimap<K, V> {