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...
public final class Doubles extends DoublesMethodsForWeb {
  private Doubles() {}
  /**
   * The number of bytes required to represent a primitive {@code double} value.
   *
   * <p><b>Java 8 users:</b> use {@link Double#BYTES} instead.
   *
   * @since 10.0
   */
  public static final int BYTES = Double.SIZE / Byte.SIZE;
  /**
   * Returns a hash code for {@code value}; equal to the result of invoking {@code ((Double)
   * value).hashCode()}.
   *
   * <p><b>Java 8 users:</b> use {@link Double#hashCode(double)} instead.
   *
   * @param value a primitive {@code double} value
   * @return a hash code for the value
   */