I just wrote a new {{KeyDescription}} template that combines data from the Wikibase entities with the template parameters. The template allows gradual migration of the data from parameters into Wikibase entities, and highlights the differences. Once switched, it will add the template to a number of tracking categories to allow us to quickly find when they do not match, so that data can be updated. It also adds an [edit] link to each property. The edit link takes users to the entity page, where they would have to hit [edit] again to actually modify it. (eventually there is a hope of making a JS wiki gadget that would do it in place, without leaving the page).
Once this template is in place, we can encourage 3rd party consumers of this data to switch to using wikibase directly. After giving them some time, we can simply remove the template parameters.
P.S. The {{ValueDescription}} template will be almost identical to {{KeyDescription}}, and would require minimal additional work to add it.
{{KeyDescription}} has been migrated. The only difference is a tiny pencil edit icon next to description. If description in wiki page is different from the data item, it will also show another red pencil icon to edit the wiki page itself.
Now that all {{Description}}-related templates have been migrated, and iD is using data items directly, I think we can slowly start removing duplicate information from the wiki pages themselves. TagInfo still uses description and onNone/onWay/onRelation parameters, e.g. see building taginfo page, but I think we can start removing other parameters like lang=, status=, etc. Does anyone know if any of them should be kept for any other reason? Thx!
I do not know of some application, but I could imagine there is one. Maybe send a message to the dev mailing list?
There are also some updates regarding the osmcarto map icons, but I think those are humans who copy the information from their repository one by one.