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...
transient Supplier<? extends Set<V>> factory;
    CustomSetMultimap(Map<K, Collection<V>> map, Supplier<? extends Set<V>> factory) {
      super(map);
      this.factory = checkNotNull(factory);
    }
    @Override
    Set<K> createKeySet() {
      return createMaybeNavigableKeySet();
    }
    @Override
    Map<K, Collection<V>> createAsMap() {
      return createMaybeNavigableAsMap();
    }
    @Override
    protected Set<V> createCollection() {
      return factory.get();
    }
    @Override
    <E> Collection<E> unmodifiableCollectionSubclass(Collection<E> collection) {
      if (collection instanceof NavigableSet) {
        return Sets.unmodifiableNavigableSet((NavigableSet<E>) collection);
      } else if (collection instanceof SortedSet) {
        return Collections.unmodifiableSortedSet((SortedSet<E>) collection);
      } else {
        return Collections.unmodifiableSet((Set<E>) collection);
      }
    }
    @Override
    Collection<V> wrapCollection(K key, Collection<V> collection) {