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...
new Function<K, Collection<V>>() {
              @Override
              public Collection<V> apply(K key) {
                return multimap.get(key);
              }
            });
      }
      @Override
      public boolean remove(Object o) {
        if (!contains(o)) {
          return false;
        }
        Map.Entry<?, ?> entry = (Map.Entry<?, ?>) o;
        removeValuesForKey(entry.getKey());
        return true;
      }
    }
    @SuppressWarnings("unchecked")
    @Override
    public Collection<V> get(Object key) {
      return containsKey(key) ? multimap.get((K) key) : null;
    }
    @Override
    public Collection<V> remove(Object key) {
      return containsKey(key) ? multimap.removeAll(key) : null;
    }
    @Override
    public Set<K> keySet() {
      return multimap.keySet();
    }