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 returned multimap will be serializable if the specified multimap is serializable.
   *
   * @param delegate the multimap for which an unmodifiable view is to be returned
   * @return an unmodifiable view of the specified multimap
   */
  public static <K, V> SortedSetMultimap<K, V> unmodifiableSortedSetMultimap(
      SortedSetMultimap<K, V> delegate) {
    if (delegate instanceof UnmodifiableSortedSetMultimap) {
      return delegate;
    }
    return new UnmodifiableSortedSetMultimap<>(delegate);
  }
  /**
   * Returns a synchronized (thread-safe) {@code ListMultimap} backed by the specified multimap.
   *
   * <p>You must follow the warnings described in {@link #synchronizedMultimap}.
   *
   * @param multimap the multimap to be wrapped
   * @return a synchronized view of the specified multimap
   */
  public static <K, V> ListMultimap<K, V> synchronizedListMultimap(ListMultimap<K, V> multimap) {
    return Synchronized.listMultimap(multimap, null);
  }
  /**
   * Returns an unmodifiable view of the specified {@code ListMultimap}. Query operations on the
   * returned multimap "read through" to the specified multimap, and attempts to modify the returned
   * multimap, either directly or through the multimap's views, result in an {@code
   * UnsupportedOperationException}.
   *
   * <p>The returned multimap will be serializable if the specified multimap is serializable.