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...
if (collection instanceof SortedSet) {
      return Collections.unmodifiableSortedSet((SortedSet<V>) collection);
    } else if (collection instanceof Set) {
      return Collections.unmodifiableSet((Set<V>) collection);
    } else if (collection instanceof List) {
      return Collections.unmodifiableList((List<V>) collection);
    }
    return Collections.unmodifiableCollection(collection);
  }
  /**
   * Returns an unmodifiable view of the specified collection of entries. The {@link Entry#setValue}
   * operation throws an {@link UnsupportedOperationException}. If the specified collection is a
   * {@code Set}, the returned collection is also a {@code Set}.
   *
   * @param entries the entries for which to return an unmodifiable view
   * @return an unmodifiable view of the entries
   */
  private static <K, V> Collection<Entry<K, V>> unmodifiableEntries(
      Collection<Entry<K, V>> entries) {
    if (entries instanceof Set) {
      return Maps.unmodifiableEntrySet((Set<Entry<K, V>>) entries);
    }
    return new Maps.UnmodifiableEntries<>(Collections.unmodifiableCollection(entries));
  }
  /**
   * Returns {@link ListMultimap#asMap multimap.asMap()}, with its type corrected from {@code Map<K,
   * Collection<V>>} to {@code Map<K, List<V>>}.
   *
   * @since 15.0
   */