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...
*     FIRST_LETTER_MULTIMAP = MultimapBuilder.treeKeys().arrayListValues().build();
   *     FIRST_LETTER_MULTIMAP.put('b', "anana");
   *     FIRST_LETTER_MULTIMAP.put('a', "pple");
   *     FIRST_LETTER_MULTIMAP.put('a', "sparagus");
   *     FIRST_LETTER_MULTIMAP.put('c', "arrot");
   *     FIRST_LETTER_MULTIMAP.put('c', "herry");
   * }
   * }</pre>
   *
   * @since 21.0
   */
  public static <T, K, V, M extends Multimap<K, V>> Collector<T, ?, M> toMultimap(
      java.util.function.Function<? super T, ? extends K> keyFunction,
      java.util.function.Function<? super T, ? extends V> valueFunction,
      java.util.function.Supplier<M> multimapSupplier) {
    checkNotNull(keyFunction);
    checkNotNull(valueFunction);
    checkNotNull(multimapSupplier);
    return Collector.of(
        multimapSupplier,
        (multimap, input) -> multimap.put(keyFunction.apply(input), valueFunction.apply(input)),
        (multimap1, multimap2) -> {
          multimap1.putAll(multimap2);
          return multimap1;
        });
  }
  /**
   * Returns a {@code Collector} accumulating entries into a {@code Multimap} generated from the
   * specified supplier. Each input element is mapped to a key and a stream of values, each of which
   * are put into the resulting {@code Multimap}, in the encounter order of the stream and the