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 forEach(BiConsumer<? super K, ? super V> consumer) {
      delegate.forEach(checkNotNull(consumer));
    }
    @Override
    public Collection<V> get(K key) {
      return unmodifiableValueCollection(delegate.get(key));
    }
    @Override
    public Multiset<K> keys() {
      Multiset<K> result = keys;
      if (result == null) {
        keys = result = Multisets.unmodifiableMultiset(delegate.keys());
      }
      return result;
    }
    @Override
    public Set<K> keySet() {
      Set<K> result = keySet;
      if (result == null) {
        keySet = result = Collections.unmodifiableSet(delegate.keySet());
      }
      return result;
    }
    @Override
    public boolean put(K key, V value) {
      throw new UnsupportedOperationException();
    }
    @Override