This has been implemented in the patch for #30937.
When a link is not previewable it will get a cursor:not-allowed style and wp.a11y.speak() will be called to accessibly notify that it cannot be live previewed. Likewise, when attempting to submit a form that is not live-previewable (if it uses a POST method or has an action that points off of the site) there will be be the same cursor:not-allowed style added to the form and all of its inputs with a similar non-previewable message output via wp.a11y.speak().
Please test: grunt patch:https://github.com/xwp/wordpress-develop/pull/161