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 final class KeySet<K> extends IndexedImmutableSet<K> {
    private final RegularImmutableMap<K, ?> map;
    KeySet(RegularImmutableMap<K, ?> map) {
      this.map = map;
    }
    @Override
    K get(int index) {
      return map.entries[index].getKey();
    }
    @Override
    public boolean contains(Object object) {
      return map.containsKey(object);
    }
    @Override
    boolean isPartialView() {
      return true;
    }
    @Override
    public int size() {
      return map.size();
    }
  }
  @Override
  ImmutableCollection<V> createValues() {
    return new Values<>(this);
  }
  @GwtCompatible(emulated = true)