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...
array[i] = ((Number) checkNotNull(boxedArray[i])).doubleValue();
    }
    return array;
  }
  /**
   * Returns a fixed-size list backed by the specified array, similar to {@link
   * Arrays#asList(Object[])}. The list supports {@link List#set(int, Object)}, but any attempt to
   * set a value to {@code null} will result in a {@link NullPointerException}.
   *
   * <p>The returned list maintains the values, but not the identities, of {@code Double} objects
   * written to or read from it. For example, whether {@code list.get(0) == list.get(0)} is true for
   * the returned list is unspecified.
   *
   * <p>The returned list may have unexpected behavior if it contains {@code NaN}, or if {@code NaN}
   * is used as a parameter to any of its methods.
   *
   * <p><b>Note:</b> when possible, you should represent your data as an {@link
   * ImmutableDoubleArray} instead, which has an {@link ImmutableDoubleArray#asList asList} view.
   *
   * @param backingArray the array to back the list
   * @return a list view of the array
   */
  public static List<Double> asList(double... backingArray) {
    if (backingArray.length == 0) {
      return Collections.emptyList();
    }
    return new DoubleArrayAsList(backingArray);
  }
  @GwtCompatible