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...
}
    @Override
    public void clear() {
      throw new UnsupportedOperationException();
    }
    @Override
    public Map<K, Collection<V>> asMap() {
      Map<K, Collection<V>> result = map;
      if (result == null) {
        result =
            map =
                Collections.unmodifiableMap(
                    Maps.transformValues(
                        delegate.asMap(),
                        new Function<Collection<V>, Collection<V>>() {
                          @Override
                          public Collection<V> apply(Collection<V> collection) {
                            return unmodifiableValueCollection(collection);
                          }
                        }));
      }
      return result;
    }
    @Override
    public Collection<Entry<K, V>> entries() {
      Collection<Entry<K, V>> result = entries;
      if (result == null) {
        entries = result = unmodifiableEntries(delegate.entries());
      }
      return result;