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 boolean contains(@Nullable Object object) {
    return object != null && Iterators.contains(iterator(), object);
  }
  @Override
  boolean isPartialView() {
    return true;
  }
  @Override
  public ImmutableList<V> asList() {
    final ImmutableList<Entry<K, V>> entryList = map.entrySet().asList();
    return new ImmutableAsList<V>() {
      @Override
      public V get(int index) {
        return entryList.get(index).getValue();
      }
      @Override
      ImmutableCollection<V> delegateCollection() {
        return ImmutableMapValues.this;
      }
    };
  }
  @GwtIncompatible 
  @Override
  public void forEach(Consumer<? super V> action) {
    checkNotNull(action);
    map.forEach((k, v) -> action.accept(v));
  }
}