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...
return (target instanceof Double)
          && Doubles.indexOf(array, (Double) target, start, end) != -1;
    }
    @Override
    public int indexOf(Object target) {
      
      if (target instanceof Double) {
        int i = Doubles.indexOf(array, (Double) target, start, end);
        if (i >= 0) {
          return i - start;
        }
      }
      return -1;
    }
    @Override
    public int lastIndexOf(Object target) {
      
      if (target instanceof Double) {
        int i = Doubles.lastIndexOf(array, (Double) target, start, end);
        if (i >= 0) {
          return i - start;
        }
      }
      return -1;
    }
    @Override
    public Double set(int index, Double element) {
      checkElementIndex(index, size());
      double oldValue = array[start + index];