I guess it’s more a recommendation than a restriction!!
Your App will work fine when using it on production, but you’ll need to keep a Webserver running for it, of course.
But I believe the biggest concern here (and probably the reason of this production warning) is the Stores Review Approval (Apple and Google).
Their guidelines are usually updated, but usually have a topic requiring code content to run directly on the App, instead consuming from external source.
Your whole App will be a big “iframe” content, always depending on external server which may cause problems to performance, getting denied to be published.
Check this discussion on the Github: Who is using server.url in production? · ionic-team/capacitor · Discussion #4080 · GitHub