This is a regression from the older Active Library concept, so I'm marking it a bug.
See also https://bugs.launchpad.net/kicad/+bug/1763200 (same for Symbol Viewer).
The existing filter box now handles library names (even when the library is empty).
It still might be nice to have a specific widget for this, but I'm moving it back to Wishlist for that.
Bug watches keep track of this bug in other bug trackers.
See also https:/ /bugs.launchpad .net/kicad/ +bug/1763200 (same for Symbol Viewer).