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...
private void writeObject(ObjectOutputStream stream) throws IOException {
      stream.defaultWriteObject();
      stream.writeObject(factory);
      stream.writeObject(backingMap());
    }
    @GwtIncompatible 
    @SuppressWarnings("unchecked") 
    private void readObject(ObjectInputStream stream) throws IOException, ClassNotFoundException {
      stream.defaultReadObject();
      factory = (Supplier<? extends Collection<V>>) stream.readObject();
      Map<K, Collection<V>> map = (Map<K, Collection<V>>) stream.readObject();
      setMap(map);
    }
    @GwtIncompatible 
    private static final long serialVersionUID = 0;
  }
  /**
   * Creates a new {@code ListMultimap} that uses the provided map and factory. It can generate a
   * multimap based on arbitrary {@link Map} and {@link List} classes.
   *
   * <p>The {@code factory}-generated and {@code map} classes determine the multimap iteration
   * order. They also specify the behavior of the {@code equals}, {@code hashCode}, and {@code
   * toString} methods for the multimap and its returned views. The multimap's {@code get}, {@code
   * removeAll}, and {@code replaceValues} methods return {@code RandomAccess} lists if the factory
   * does. However, the multimap's {@code get} method returns instances of a different class than
   * does {@code factory.get()}.
   *
   * <p>The multimap is serializable if {@code map}, {@code factory}, the lists generated by {@code
   * factory}, and the multimap contents are all serializable.
   *