This filtering was dropped in 4565cc59 (v5.3.0).
Now it's reintroduced - but inside the function, as that seems nicer.
Nit: naming and comment were updated to fit the current usage.

As the code is designed so far (in whole history probably), in order
to detect whether we need to choose a zone cut closer to the root,
we need to do something like this in lib/zonecut.c already,
instead of just during server selection.

I don't think this change can break anything.
Fetching unusable addresses from cache seems pointless,
as selection wouldn't be allowed to use them or try resolving them.