Yes, I mirror your dismay, and found the SURPRISING change MORE than merely annoying. Some Marketing IDGITS apparently decided to consider this functionality 'feature creep,' and removed it -- without providing at least a comparable full-capability management system for the removed features.
Personally, I deeply appreciated the ability to manage all my content for all my devices from ONE application. (Including the Apps, Audiobooks, Ringtones, Contacts, etc. for which they've now removed support.) And to do so with the ability to easily see all that content listed, to pick-and-choose which content I wanted downloaded, which I wanted updated, under what circumstances. Not to mention perform a FULL iPhone backup and restore with relative simplicity.
(1) There are some alternative (far less efficient) approaches helpfully outlined by others for doing what you need on the device(s) themselves, posted in other, later threads on this forum. (2) See Deploy apps in a business environment with iTunes - Apple Support for access to iTunes 12.6.3 (at least for now ...) if you want to revert to a build that Apple supplied that still has the app management features. (3) If you'd like to complain to Apple where they will actually see it, do so here and fill out the feedback form.