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...
if (values == null) {
        return 0;
      }
      int oldCount = values.size();
      if (occurrences >= oldCount) {
        values.clear();
      } else {
        Iterator<V> iterator = values.iterator();
        for (int i = 0; i < occurrences; i++) {
          iterator.next();
          iterator.remove();
        }
      }
      return oldCount;
    }
    @Override
    public void clear() {
      multimap.clear();
    }
    @Override
    public Set<K> elementSet() {
      return multimap.keySet();
    }
    @Override
    Iterator<K> elementIterator() {
      throw new AssertionError("should never be called");
    }
  }
  
  abstract static class Entries<K, V> extends AbstractCollection<Map.Entry<K, V>> {