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...
private static class FloatArrayAsList extends AbstractList<Float>
      implements RandomAccess, Serializable {
    final float[] array;
    final int start;
    final int end;
    FloatArrayAsList(float[] array) {
      this(array, 0, array.length);
    }
    FloatArrayAsList(float[] array, int start, int end) {
      this.array = array;
      this.start = start;
      this.end = end;
    }
    @Override
    public int size() {
      return end - start;
    }
    @Override
    public boolean isEmpty() {
      return false;
    }
    @Override
    public Float get(int index) {
      checkElementIndex(index, size());
      return array[start + index];
    }