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...
public static <K, V> Multimap<K, V> unmodifiableMultimap(Multimap<K, V> delegate) {
    if (delegate instanceof UnmodifiableMultimap || delegate instanceof ImmutableMultimap) {
      return delegate;
    }
    return new UnmodifiableMultimap<>(delegate);
  }
  /**
   * Simply returns its argument.
   *
   * @deprecated no need to use this
   * @since 10.0
   */
  @Deprecated
  public static <K, V> Multimap<K, V> unmodifiableMultimap(ImmutableMultimap<K, V> delegate) {
    return checkNotNull(delegate);
  }
  private static class UnmodifiableMultimap<K, V> extends ForwardingMultimap<K, V>
      implements Serializable {
    final Multimap<K, V> delegate;
    transient @Nullable Collection<Entry<K, V>> entries;
    transient @Nullable Multiset<K> keys;
    transient @Nullable Set<K> keySet;
    transient @Nullable Collection<V> values;
    transient @Nullable Map<K, Collection<V>> map;
    UnmodifiableMultimap(final Multimap<K, V> delegate) {
      this.delegate = checkNotNull(delegate);
    }
    @Override
    protected Multimap<K, V> delegate() {
      return delegate;