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...
* throwing an exception if parsing fails. Valid inputs are exactly those accepted by {@link
   * Float#valueOf(String)}, except that leading and trailing whitespace is not permitted.
   *
   * <p>This implementation is likely to be faster than {@code Float.parseFloat} if many failures
   * are expected.
   *
   * @param string the string representation of a {@code float} value
   * @return the floating point value represented by {@code string}, or {@code null} if {@code
   *     string} has a length of zero or cannot be parsed as a {@code float} value
   * @throws NullPointerException if {@code string} is {@code null}
   * @since 14.0
   */
  @Beta
  @GwtIncompatible 
  @NullableDecl
  public static Float tryParse(String string) {
    if (Doubles.FLOATING_POINT_PATTERN.matcher(string).matches()) {
      
      
      try {
        return Float.parseFloat(string);
      } catch (NumberFormatException e) {
        
        
      }
    }
    return null;
  }
}