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...
@Weak private final Multimap<K, V> multimap;
    AsMap(Multimap<K, V> multimap) {
      this.multimap = checkNotNull(multimap);
    }
    @Override
    public int size() {
      return multimap.keySet().size();
    }
    @Override
    protected Set<Entry<K, Collection<V>>> createEntrySet() {
      return new EntrySet();
    }
    void removeValuesForKey(Object key) {
      multimap.keySet().remove(key);
    }
    @WeakOuter
    class EntrySet extends Maps.EntrySet<K, Collection<V>> {
      @Override
      Map<K, Collection<V>> map() {
        return AsMap.this;
      }
      @Override
      public Iterator<Entry<K, Collection<V>>> iterator() {
        return Maps.asMapEntryIterator(
            multimap.keySet(),