After several months of sitting on my backside about this, I've decided to try and rehabilitate these two offending little tags. They look so simple in theory, don't they? You just add

disused=yes

to something to mark as it disused, and you're done, right? Wrong, because it generates inconsistencies and internal contradictions in the data if you tag a disused pub, say, like the wiki docs used to recommend:

amenity=pub
disused=yes

Oops. Logically, something can't be both currently used as a pub and currently disused. Additionally, software written without knowledge of the magic "disused=yes" or any of the many other weird and wonderful tags that share the same pattern. Which means that routing engines may route people to disused objects, and disused objects render on the map. This calls for a better approach, a backwards-compatible one: namespaces and better docs to the rescue. For ages now, folks on IRC and mappers generally have been doing something like

disused=yes
disused:amenity=pub

to prevent software that has no reason to understand new magic tagging from doing the wrong thing. I've just written it up at

http://wiki.openstreetmap.org/wiki/Key:disused
http://wiki.openstreetmap.org/wiki/Key:abandoned

with some thoughts about naming and and when a building stops being a building. It's looking good to me, anyway.

In general, tag-semantically, if you're proposing a tag like this which completely breaks the meaning of other tags or keys, you probably shouldn't. But if you want to, please consider a pattern like this to prevent bad data from happening.