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() {
      fromMultimap.clear();
    }
    @Override
    public boolean containsKey(Object key) {
      return fromMultimap.containsKey(key);
    }
    @Override
    Collection<Entry<K, V2>> createEntries() {
      return new Entries();
    }
    @Override
    Iterator<Entry<K, V2>> entryIterator() {
      return Iterators.transform(
          fromMultimap.entries().iterator(), Maps.<K, V1, V2>asEntryToEntryFunction(transformer));
    }
    @Override
    public Collection<V2> get(final K key) {
      return transform(key, fromMultimap.get(key));
    }
    @Override
    public boolean isEmpty() {
      return fromMultimap.isEmpty();
    }
    @Override
    Set<K> createKeySet() {
      return fromMultimap.keySet();
    }
    @Override