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...
Multiset<K> createKeys() {
      return fromMultimap.keys();
    }
    @Override
    public boolean put(K key, V2 value) {
      throw new UnsupportedOperationException();
    }
    @Override
    public boolean putAll(K key, Iterable<? extends V2> values) {
      throw new UnsupportedOperationException();
    }
    @Override
    public boolean putAll(Multimap<? extends K, ? extends V2> multimap) {
      throw new UnsupportedOperationException();
    }
    @SuppressWarnings("unchecked")
    @Override
    public boolean remove(Object key, Object value) {
      return get((K) key).remove(value);
    }
    @SuppressWarnings("unchecked")
    @Override
    public Collection<V2> removeAll(Object key) {
      return transform((K) key, fromMultimap.removeAll(key));
    }
    @Override
    public Collection<V2> replaceValues(K key, Iterable<? extends V2> values) {
      throw new UnsupportedOperationException();
    }
    @Override