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...
lenientFormat("min (%s) must be less than or equal to max (%s)", min, max));
  }
  /**
   * Returns the values from each provided array combined into a single array. For example, {@code
   * concat(new double[] {a, b}, new double[] {}, new double[] {c}} returns the array {@code {a, b,
   * c}}.
   *
   * @param arrays zero or more {@code double} arrays
   * @return a single array containing all the values from the source arrays, in order
   */
  public static double[] concat(double[]... arrays) {
    int length = 0;
    for (double[] array : arrays) {
      length += array.length;
    }
    double[] result = new double[length];
    int pos = 0;
    for (double[] array : arrays) {
      System.arraycopy(array, 0, result, pos, array.length);
      pos += array.length;
    }
    return result;
  }
  private static final class DoubleConverter extends Converter<String, Double>
      implements Serializable {
    static final DoubleConverter INSTANCE = new DoubleConverter();
    @Override
    protected Double doForward(String value) {
      return Double.valueOf(value);
    }