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...
for (int i = end - 1; i >= start; i--) {
      if (array[i] == target) {
        return i;
      }
    }
    return -1;
  }
  /**
   * Returns the least value present in {@code array}, using the same rules of comparison as {@link
   * Math#min(float, float)}.
   *
   * @param array a <i>nonempty</i> array of {@code float} values
   * @return the value present in {@code array} that is less than or equal to every other value in
   *     the array
   * @throws IllegalArgumentException if {@code array} is empty
   */
  @GwtIncompatible(
      "Available in GWT! Annotation is to avoid conflict with GWT specialization of base class.")
  public static float min(float... array) {
    checkArgument(array.length > 0);
    float min = array[0];
    for (int i = 1; i < array.length; i++) {
      min = Math.min(min, array[i]);
    }
    return min;
  }
  /**
   * Returns the greatest value present in {@code array}, using the same rules of comparison as
   * {@link Math#max(float, float)}.
   *