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...
protected Double doForward(String value) {
      return Double.valueOf(value);
    }
    @Override
    protected String doBackward(Double value) {
      return value.toString();
    }
    @Override
    public String toString() {
      return "Doubles.stringConverter()";
    }
    private Object readResolve() {
      return INSTANCE;
    }
    private static final long serialVersionUID = 1;
  }
  /**
   * Returns a serializable converter object that converts between strings and doubles using {@link
   * Double#valueOf} and {@link Double#toString()}.
   *
   * @since 16.0
   */
  @Beta
  public static Converter<String, Double> stringConverter() {
    return DoubleConverter.INSTANCE;
  }
  /**
   * Returns an array containing the same values as {@code array}, but guaranteed to be of a
   * specified minimum length. If {@code array} already has a length of at least {@code minLength},
   * it is returned directly. Otherwise, a new array of size {@code minLength + padding} is