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 (key.equals(candidateKey)) {
        return entry.getValue();
      }
    }
    return null;
  }
  @Override
  public void forEach(BiConsumer<? super K, ? super V> action) {
    checkNotNull(action);
    for (Entry<K, V> entry : entries) {
      action.accept(entry.getKey(), entry.getValue());
    }
  }
  @Override
  public int size() {
    return entries.length;
  }
  @Override
  boolean isPartialView() {
    return false;
  }
  @Override
  ImmutableSet<Entry<K, V>> createEntrySet() {
    return new ImmutableMapEntrySet.RegularEntrySet<>(this, entries);
  }
  @Override
  ImmutableSet<K> createKeySet() {
    return new KeySet<>(this);
  }
  @GwtCompatible(emulated = true)