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>Changes in the underlying multimap are reflected in this view. Conversely, this view
   * supports removal operations, and these are reflected in the underlying multimap.
   *
   * <p>It's acceptable for the underlying multimap to contain null keys and null values provided
   * that the transformer is capable of accepting null inputs. The transformed multimap might
   * contain null values if the transformer sometimes gives a null result.
   *
   * <p>The returned multimap is not thread-safe or serializable, even if the underlying multimap
   * is.
   *
   * <p>The transformer is applied lazily, invoked when needed. This is necessary for the returned
   * multimap to be a view, but it means that the transformer will be applied many times for bulk
   * operations like {@link Multimap#containsValue} and {@link Object#toString}. For this to perform
   * well, {@code transformer} should be fast. To avoid lazy evaluation when the returned multimap
   * doesn't need to be a view, copy the returned multimap into a new multimap of your choosing.
   *
   * <p><b>Warning:</b> This method assumes that for any instance {@code k} of {@code
   * EntryTransformer} key type {@code K}, {@code k.equals(k2)} implies that {@code k2} is also of
   * type {@code K}. Using an {@code EntryTransformer} key type for which this may not hold, such as
   * {@code ArrayList}, may risk a {@code ClassCastException} when calling methods on the
   * transformed multimap.
   *
   * @since 7.0
   */
  public static <K, V1, V2> ListMultimap<K, V2> transformEntries(
      ListMultimap<K, V1> fromMap, EntryTransformer<? super K, ? super V1, V2> transformer) {
    return new TransformedEntriesListMultimap<>(fromMap, transformer);
  }
  private static class TransformedEntriesMultimap<K, V1, V2> extends AbstractMultimap<K, V2> {