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 put(K key, V value) {
      throw new UnsupportedOperationException();
    }
    @Override
    public boolean putAll(K key, Iterable<? extends V> values) {
      throw new UnsupportedOperationException();
    }
    @Override
    public boolean putAll(Multimap<? extends K, ? extends V> multimap) {
      throw new UnsupportedOperationException();
    }
    @Override
    public Set<V> replaceValues(K key, Iterable<? extends V> values) {
      throw new UnsupportedOperationException();
    }
    @Override
    public boolean remove(Object key, Object value) {
      return map.entrySet().remove(Maps.immutableEntry(key, value));
    }
    @Override
    public Set<V> removeAll(Object key) {
      Set<V> values = new HashSet<V>(2);
      if (!map.containsKey(key)) {
        return values;
      }
      values.add(map.remove(key));
      return values;