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...
@Beta
  @SuppressWarnings("unchecked")
  
  public static <K, V> Map<K, List<V>> asMap(ListMultimap<K, V> multimap) {
    return (Map<K, List<V>>) (Map<K, ?>) multimap.asMap();
  }
  /**
   * Returns {@link SetMultimap#asMap multimap.asMap()}, with its type corrected from {@code Map<K,
   * Collection<V>>} to {@code Map<K, Set<V>>}.
   *
   * @since 15.0
   */
  @Beta
  @SuppressWarnings("unchecked")
  
  public static <K, V> Map<K, Set<V>> asMap(SetMultimap<K, V> multimap) {
    return (Map<K, Set<V>>) (Map<K, ?>) multimap.asMap();
  }
  /**
   * Returns {@link SortedSetMultimap#asMap multimap.asMap()}, with its type corrected from {@code
   * Map<K, Collection<V>>} to {@code Map<K, SortedSet<V>>}.
   *
   * @since 15.0
   */
  @Beta
  @SuppressWarnings("unchecked")
  
  public static <K, V> Map<K, SortedSet<V>> asMap(SortedSetMultimap<K, V> multimap) {
    return (Map<K, SortedSet<V>>) (Map<K, ?>) multimap.asMap();
  }