Filter out of stock attributes

Not exactly sure what you mean. You can set individual variations to be out of stock – setting it just at the attribute level isn’t possible though by default (unless you only have one attribute on the product, then just switch that variation to be out of stock).

Sorry, I meant in the layered navigation. I think what I’d actually need is a Layered navigation widget that filters by variations / stock, and not by “raw” attributes (which is also good but not optimal for many cases, e. g. clothing, shoes, etc.).

Attributes should only show up in the layered nav if there are products available using the attribute.

Are you hiding out of stock products?

Nah, we have a clothing shop, we need our clients to see our products even if unavailable when they navigate through categories, but when filtering available sizes for example, it’s really frustrating to show out of stock sizes.

Completely hiding out of stock products, if that filters out out of stock variables in layered nav, would be a partial solution though…

Well, by default you can only have it one way or the other. Either out of stock products are available and filterable – or they are hidden an not filterable.

It will take some customization to get half and half.