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...
final class ImmutableMapValues<K, V> extends ImmutableCollection<V> {
  private final ImmutableMap<K, V> map;
  ImmutableMapValues(ImmutableMap<K, V> map) {
    this.map = map;
  }
  @Override
  public int size() {
    return map.size();
  }
  @Override
  public UnmodifiableIterator<V> iterator() {
    return new UnmodifiableIterator<V>() {
      final UnmodifiableIterator<Entry<K, V>> entryItr = map.entrySet().iterator();
      @Override
      public boolean hasNext() {
        return entryItr.hasNext();
      }
      @Override
      public V next() {
        return entryItr.next().getValue();
      }
    };
  }
  @Override
  public Spliterator<V> spliterator() {
    return CollectSpliterators.map(map.entrySet().spliterator(), Entry::getValue);
  }