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...
table[tableIndex] = newEntry;
      entries[entryIndex] = newEntry;
      int bucketSize = checkNoConflictInKeyBucket(key, newEntry, existing);
      if (bucketSize > MAX_HASH_BUCKET_LENGTH) {
        
        
        return JdkBackedImmutableMap.create(n, entryArray);
      }
    }
    return new RegularImmutableMap<>(entries, table, mask);
  }
  
  static <K, V> ImmutableMapEntry<K, V> makeImmutable(Entry<K, V> entry, K key, V value) {
    boolean reusable =
        entry instanceof ImmutableMapEntry && ((ImmutableMapEntry<K, V>) entry).isReusable();
    return reusable ? (ImmutableMapEntry<K, V>) entry : new ImmutableMapEntry<K, V>(key, value);
  }
  
  static <K, V> ImmutableMapEntry<K, V> makeImmutable(Entry<K, V> entry) {
    return makeImmutable(entry, entry.getKey(), entry.getValue());
  }
  private RegularImmutableMap(Entry<K, V>[] entries, ImmutableMapEntry<K, V>[] table, int mask) {
    this.entries = entries;
    this.table = table;
    this.mask = mask;
  }
  /**
   * @return number of entries in this bucket
   * @throws IllegalArgumentException if another entry in the bucket has the same key
   */