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 (collection instanceof NavigableSet) {
        return new WrappedNavigableSet(key, (NavigableSet<V>) collection, null);
      } else if (collection instanceof SortedSet) {
        return new WrappedSortedSet(key, (SortedSet<V>) collection, null);
      } else {
        return new WrappedSet(key, (Set<V>) collection);
      }
    }
    
    @GwtIncompatible 
    private void writeObject(ObjectOutputStream stream) throws IOException {
      stream.defaultWriteObject();
      stream.writeObject(factory);
      stream.writeObject(backingMap());
    }
    @GwtIncompatible 
    @SuppressWarnings("unchecked") 
    private void readObject(ObjectInputStream stream) throws IOException, ClassNotFoundException {
      stream.defaultReadObject();
      factory = (Supplier<? extends Set<V>>) stream.readObject();
      Map<K, Collection<V>> map = (Map<K, Collection<V>>) stream.readObject();
      setMap(map);
    }
    @GwtIncompatible 
    private static final long serialVersionUID = 0;
  }
  /**
   * Creates a new {@code SortedSetMultimap} that uses the provided map and factory. It can generate
   * a multimap based on arbitrary {@link Map} and {@link SortedSet} classes.
   *