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...
public boolean putAll(K key, Iterable<? extends V> values) {
      throw new UnsupportedOperationException();
    }
    @Override
    public boolean putAll(Multimap<? extends K, ? extends V> multimap) {
      throw new UnsupportedOperationException();
    }
    @Override
    public boolean remove(Object key, Object value) {
      throw new UnsupportedOperationException();
    }
    @Override
    public Collection<V> removeAll(Object key) {
      throw new UnsupportedOperationException();
    }
    @Override
    public Collection<V> replaceValues(K key, Iterable<? extends V> values) {
      throw new UnsupportedOperationException();
    }
    @Override
    public Collection<V> values() {
      Collection<V> result = values;
      if (result == null) {
        values = result = Collections.unmodifiableCollection(delegate.values());
      }
      return result;
    }
    private static final long serialVersionUID = 0;
  }
  private static class UnmodifiableListMultimap<K, V> extends UnmodifiableMultimap<K, V>