Why restrict to URN when you can use a URI that looks similar to a URL but doesn't have to be accessible. I've used URIs for partner supplied identifiers. They're also used with GraphQL as resource ids.
For widely standardized things URNs make complete sense e.g. ISBN for books.
Well I want to refer to things that are widely standardized within countries: Business registration numbers and bank account numbers.
However when you go international, how should you refer to "business registration number in Norway"?
Perhaps you encode that as "NO" but then you hit Denmark which has not 1 but 5 business registries....
The ONLY thing missing is really a standard way to refer to "the registry in which the ID is registered". Like "urn:isbn:" does.
I would love to have the same e.g. for referring to a business registration number or bank account number.
Making my own URLs to refer to a business registration number that users of the system provides...not sure in what way I would do that or what value it would give.
So URIs are almost the opposite. If you think of the 'path' as the thing being registered, then the hostname part (which aren't resolvable hosts but can follow domain ownership convention) is where it's registered. (Could also use up to a common path prefix.) What we don't have are standards for the 'path' parts for various usages.
For widely standardized things URNs make complete sense e.g. ISBN for books.