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 int compare(float[] left, float[] right) {
      int minLength = Math.min(left.length, right.length);
      for (int i = 0; i < minLength; i++) {
        int result = Float.compare(left[i], right[i]);
        if (result != 0) {
          return result;
        }
      }
      return left.length - right.length;
    }
    @Override
    public String toString() {
      return "Floats.lexicographicalComparator()";
    }
  }
  /**
   * Sorts the elements of {@code array} in descending order.
   *
   * <p>Note that this method uses the total order imposed by {@link Float#compare}, which treats
   * all NaN values as equal and 0.0 as greater than -0.0.
   *
   * @since 23.1
   */
  public static void sortDescending(float[] array) {
    checkNotNull(array);
    sortDescending(array, 0, array.length);
  }
  /**
   * Sorts the elements of {@code array} between {@code fromIndex} inclusive and {@code toIndex}