Management of apps, books, and tones has been removed from iTunes 12.7. See Changes in the new iTunes - Apple Support for details. The intention is clearly that management of these generally takes place on the device.
See the Manually add items from your computer section of Manually manage content on your iPhone, iPad, or iPod touch using iTunes - Apple Support for the control that remains. This can be used to copy apps (including apps that have been withdrawn from the store), ePubs, iBooks, PDFs, and tones from your computer to your device. The File Sharing feature for copying data between apps and the computer is still there. You can also use iCloud Drive, or Dropbox to move content to and from the device.
A long time criticism of iTunes, at least from some quarters, is that has been that it has become bloated. Perhaps this is an attempt to address that. I let both my iOS devices update automatically when there is an updated app, and I'm not interested in hanging on to old apps that a developer has withdrawn, nor do I need to rearrange my apps often, so while the change is drastic, I don't think it really affects me much. I'm more irritated by the changes to iTunes U, and iBooks in iTunes for Windows.
See Deploy apps in a business environment with iTunes - Apple Support for access to iTunes 12.6.3 if you want to revert to a build with app management features. Windows users need to uninstall the current copy of the iTunes application first. See Empty/corrupt iTunes library after upgrade/crash for steps to revert to a pre-iTunes 12.7 version of your iTunes database.
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