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 Set<V> get(final K key) {
      return new Sets.ImprovedAbstractSet<V>() {
        @Override
        public Iterator<V> iterator() {
          return new Iterator<V>() {
            int i;
            @Override
            public boolean hasNext() {
              return (i == 0) && map.containsKey(key);
            }
            @Override
            public V next() {
              if (!hasNext()) {
                throw new NoSuchElementException();
              }
              i++;
              return map.get(key);
            }
            @Override
            public void remove() {
              checkRemove(i == 1);
              i = -1;
              map.remove(key);
            }
          };
        }
        @Override
        public int size() {
          return map.containsKey(key) ? 1 : 0;
        }