[fanf2]

The most common place I have seen them is in XML namespaces, eg in Jabber like xmlns='urn:ietf:params:xml:ns:xmpp-streams'

When a protocol ID is a URI it is common to use a URL rather than a URN so that the ID can serve as a link to its own documentation.

There is a bonkers DNS record called NAPTR https://en.wikipedia.org/wiki/NAPTR_record which was designed to be used to make the URN mapping database mentioned towards the end of your quote, using a combination of regex rewriting and chasing around the DNS. I get the impression NAPTR was never really used for resolving URNs but it has a second life for mapping phone numbers to network services.