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...
/**
   * Returns a hash code for {@code value}; equal to the result of invoking {@code ((Float)
   * value).hashCode()}.
   *
   * <p><b>Java 8 users:</b> use {@link Float#hashCode(float)} instead.
   *
   * @param value a primitive {@code float} value
   * @return a hash code for the value
   */
  public static int hashCode(float value) {
    
    return ((Float) value).hashCode();
  }
  /**
   * Compares the two specified {@code float} values using {@link Float#compare(float, float)}. You
   * may prefer to invoke that method directly; this method exists only for consistency with the
   * other utilities in this package.
   *
   * <p><b>Note:</b> this method simply delegates to the JDK method {@link Float#compare}. It is
   * provided for consistency with the other primitive types, whose compare methods were not added
   * to the JDK until JDK 7.
   *
   * @param a the first {@code float} to compare
   * @param b the second {@code float} to compare
   * @return the result of invoking {@link Float#compare(float, float)}
   */
  public static int compare(float a, float b) {
    return Float.compare(a, b);
  }
  /**