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...
*
   * @deprecated no need to use this
   * @since 10.0
   */
  @Deprecated
  public static <K, V> SetMultimap<K, V> unmodifiableSetMultimap(
      ImmutableSetMultimap<K, V> delegate) {
    return checkNotNull(delegate);
  }
  /**
   * Returns a synchronized (thread-safe) {@code SortedSetMultimap} backed by the specified
   * multimap.
   *
   * <p>You must follow the warnings described in {@link #synchronizedMultimap}.
   *
   * <p>The returned multimap will be serializable if the specified multimap is serializable.
   *
   * @param multimap the multimap to be wrapped
   * @return a synchronized view of the specified multimap
   */
  public static <K, V> SortedSetMultimap<K, V> synchronizedSortedSetMultimap(
      SortedSetMultimap<K, V> multimap) {
    return Synchronized.sortedSetMultimap(multimap, null);
  }
  /**
   * Returns an unmodifiable view of the specified {@code SortedSetMultimap}. 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}.
   *