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...
* Returns {@code true} if {@code value} represents a real number. This is equivalent to, but not
   * necessarily implemented as, {@code !(Float.isInfinite(value) || Float.isNaN(value))}.
   *
   * <p><b>Java 8 users:</b> use {@link Float#isFinite(float)} instead.
   *
   * @since 10.0
   */
  public static boolean isFinite(float value) {
    return NEGATIVE_INFINITY < value && value < POSITIVE_INFINITY;
  }
  /**
   * Returns {@code true} if {@code target} is present as an element anywhere in {@code array}. Note
   * that this always returns {@code false} when {@code target} is {@code NaN}.
   *
   * @param array an array of {@code float} values, possibly empty
   * @param target a primitive {@code float} value
   * @return {@code true} if {@code array[i] == target} for some value of {@code i}
   */
  public static boolean contains(float[] array, float target) {
    for (float value : array) {
      if (value == target) {
        return true;
      }
    }
    return false;
  }
  /**
   * Returns the index of the first appearance of the value {@code target} in {@code array}. Note
   * that this always returns {@code -1} when {@code target} is {@code NaN}.
   *