Why is a URL mandatory for a related version?

As all metadata should be self-contained, please add a persistent URL, e.g. \relatedversion{A full version of the paper is available at \url{...}.}. This also simplifies the access for all readers. Additional to the URL, you might add a reference (\cite{...}).

Metadata should be self-contained as they are not only part of the document / PDF but also extracted and stored in a machine-readable format along with the actual document.

Please note: As hosting on a (personal or university) webpage or in cloud storage is not really sufficient for durable / persistent file storage, we highly recommend to publish your document in a reliable repository like arXiv or HAL.