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 static final class Values<K, V> extends ImmutableList<V> {
    final RegularImmutableMap<K, V> map;
    Values(RegularImmutableMap<K, V> map) {
      this.map = map;
    }
    @Override
    public V get(int index) {
      return map.entries[index].getValue();
    }
    @Override
    public int size() {
      return map.size();
    }
    @Override
    boolean isPartialView() {
      return true;
    }
  }
  
  
  private static final long serialVersionUID = 0;
}