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...
public int size() {
    return map.size();
  }
  @Override
  public UnmodifiableIterator<K> iterator() {
    return map.keyIterator();
  }
  @Override
  public Spliterator<K> spliterator() {
    return map.keySpliterator();
  }
  @Override
  public boolean contains(@Nullable Object object) {
    return map.containsKey(object);
  }
  @Override
  K get(int index) {
    return map.entrySet().asList().get(index).getKey();
  }
  @Override
  public void forEach(Consumer<? super K> action) {
    checkNotNull(action);
    map.forEach((k, v) -> action.accept(k));
  }
  @Override
  boolean isPartialView() {
    return true;
  }
}