Loss in navigability of one's collection with update to iTunes 1.0.0.426
FYI, the feature I described was called the 3 column browser in iTunes.

According to other users, it has been removed in transitioning to Apple Music. Best we can do to get it back is submit feedback requesting that, here: https://www.apple.com/feedback/macos.html

As for searching your library, you can recover that by enabling the "Filter" field under View.