Was it this thread : Sources and Citations: some thoughts ? The key:value possibility seems to come from the GEDCOM 7 specification, but it is only a recommendation so I doubt any software actually does something useful with it.
I guess it depends on sources.
Most of my sources come from French archives that actually stores the original physical data. Barring any catastrophic event, these will still exist in decades (or if they don’t, then the citation will be as obsolete as the URL).
As for URL changes, it is not really a problem when organizations generate ARKs. They update the base URL themselves in the NAAN registry and the resolver takes you to the new place. Well, I guess this is more saying that in this case ARK, not URL, is part of the citation, and the URL can be easily reconstructed. In a corner of my brain, with a million other projects, is the idea to build an ARK resolver addon for Gramps (along with a citation plug-in to generate the current URL for reports maybe ?).
All that to say that the URL should not be the only identifying part of the citation when possible, but it can be a very useful resource
For now, I do store the URL in a “Link” note, and I am toying with the idea of storing the ARK in an attribute. I have not yet tested reports and exports.