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...
multimap.entries().forEach(entry -> consumer.accept(entry.getKey()));
    }
    @Override
    int distinctElements() {
      return multimap.asMap().size();
    }
    @Override
    public int size() {
      return multimap.size();
    }
    @Override
    public boolean contains(@Nullable Object element) {
      return multimap.containsKey(element);
    }
    @Override
    public Iterator<K> iterator() {
      return Maps.keyIterator(multimap.entries().iterator());
    }
    @Override
    public int count(@Nullable Object element) {
      Collection<V> values = Maps.safeGet(multimap.asMap(), element);
      return (values == null) ? 0 : values.size();
    }
    @Override
    public int remove(@Nullable Object element, int occurrences) {
      checkNonnegative(occurrences, "occurrences");
      if (occurrences == 0) {
        return count(element);
      }
      Collection<V> values = Maps.safeGet(multimap.asMap(), element);