Once $wgUrlShortenerReadOnly to deactivated, users will only be prevented from creating short urls on non-meta wikis because they lack the user rights needed. Currently, the message that would be shown is the default PermissionsError based on the lack of rights, and the api will only return the apierror-permissiondenied message. Instead, UrlShortener should introduce two new messages (for the special page and the api) that serve the same function (explaining the lack of user rights) but that can be overridden with wikimedia messages to explain that meta should be used.
I propose the following keys: urlshortener-badaccessgroups and apierror-urlshortener-permissiondenied