Both on staff interface and opac, the availability filter and facet aren't working. This has been an issue for some time.
The feature should be removed until it is fixed.
Maybe it would be good to have someone dive into the code and see how it currently works. I was told in the past that it works ok for some libraries but not in the past. Hard to tell without knowing what is actually going on.
It would be a useful filter if it did work correctly.
Bug 11677 is now resolved, I think we can close this one as invalid.