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 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
   * #synchronizedListMultimap}.
   *
   * <p>Call this method only when the simpler methods {@link ArrayListMultimap#create()} and {@link
   * LinkedListMultimap#create()} won't suffice.
   *
   * <p>Note: the multimap assumes complete ownership over of {@code map} and the lists returned by
   * {@code factory}. Those objects should not be manually updated, they should be empty when
   * provided, 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 lists that will each hold all values for a given key
   * @throws IllegalArgumentException if {@code map} is not empty
   */
  public static <K, V> ListMultimap<K, V> newListMultimap(
      Map<K, Collection<V>> map, final Supplier<? extends List<V>> factory) {
    return new CustomListMultimap<>(map, factory);
  }
  private static class CustomListMultimap<K, V> extends AbstractListMultimap<K, V> {
    transient Supplier<? extends List<V>> factory;
    CustomListMultimap(Map<K, Collection<V>> map, Supplier<? extends List<V>> factory) {
      super(map);
      this.factory = checkNotNull(factory);
    }
    @Override
    Set<K> createKeySet() {
      return createMaybeNavigableKeySet();
    }