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 SetMultimap<K, V>, Serializable {
    final Map<K, V> map;
    MapMultimap(Map<K, V> map) {
      this.map = checkNotNull(map);
    }
    @Override
    public int size() {
      return map.size();
    }
    @Override
    public boolean containsKey(Object key) {
      return map.containsKey(key);
    }
    @Override
    public boolean containsValue(Object value) {
      return map.containsValue(value);
    }
    @Override
    public boolean containsEntry(Object key, Object value) {
      return map.entrySet().contains(Maps.immutableEntry(key, value));
    }
    @Override