While in created Playlists/Subfolders (i.e. Starred, Running Music, Workout Music) there should be a search feature. These lists can get rather large and it would be nice to find a specific song quickly.
Filtering within playlists is already implemented: on iOS/Android, swipe to the top of your playlist to show the Filter bar, where you can type in the name of the desired song. On desktop, hit Ctrl (Cmd if on Mac) + F to show the Filter bar.