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...
implements ListMultimap<K, V> {
    UnmodifiableListMultimap(ListMultimap<K, V> delegate) {
      super(delegate);
    }
    @Override
    public ListMultimap<K, V> delegate() {
      return (ListMultimap<K, V>) super.delegate();
    }
    @Override
    public List<V> get(K key) {
      return Collections.unmodifiableList(delegate().get(key));
    }
    @Override
    public List<V> removeAll(Object key) {
      throw new UnsupportedOperationException();
    }
    @Override
    public List<V> replaceValues(K key, Iterable<? extends V> values) {
      throw new UnsupportedOperationException();
    }
    private static final long serialVersionUID = 0;
  }
  private static class UnmodifiableSetMultimap<K, V> extends UnmodifiableMultimap<K, V>