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...
FilteredMultimap<K, V> multimap, Predicate<? super Entry<K, V>> entryPredicate) {
    Predicate<Entry<K, V>> predicate =
        Predicates.<Entry<K, V>>and(multimap.entryPredicate(), entryPredicate);
    return new FilteredEntryMultimap<>(multimap.unfiltered(), predicate);
  }
  /**
   * Support removal operations when filtering a filtered multimap. Since a filtered multimap has
   * iterators that don't support remove, passing one to the FilteredEntryMultimap constructor would
   * lead to a multimap whose removal operations would fail. This method combines the predicates to
   * avoid that problem.
   */
  private static <K, V> SetMultimap<K, V> filterFiltered(
      FilteredSetMultimap<K, V> multimap, Predicate<? super Entry<K, V>> entryPredicate) {
    Predicate<Entry<K, V>> predicate =
        Predicates.<Entry<K, V>>and(multimap.entryPredicate(), entryPredicate);
    return new FilteredEntrySetMultimap<>(multimap.unfiltered(), predicate);
  }
  static boolean equalsImpl(Multimap<?, ?> multimap, @Nullable Object object) {
    if (object == multimap) {
      return true;
    }
    if (object instanceof Multimap) {
      Multimap<?, ?> that = (Multimap<?, ?>) object;
      return multimap.asMap().equals(that.asMap());
    }
    return false;
  }
  
}