Both on staff interface and opac, the availability filter and facet aren't working. This has been an issue for some time. https://bugs.koha-community.org/bugzilla3/show_bug.cgi?id=11677 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.