Is it possible to load an app onto an iOS 11 device using iTunes 12.6 or earlier?
There’s no need to specifically use iTunes v12.6 if all you want is to copy the iOS apps that you already have on your Mac to an iOS device: Apple’s support articles suggest this can still be done with iTunes v12.7. However, do keep in mind this only goes for the iOS apps you currently have on your Mac, it is not possible anymore to download other apps using the new iTunes.

According to the Apple Support article “Changes in the new iTunes”, you can still copy any of the apps that you have in your iTunes Media folder to an iOS device:


  The new iTunes focuses on music, movies, TV shows, podcasts, and audiobooks. Apps for iPhone, iPad, and iPod touch are now exclusively available in the new App Store for iOS. […] You can find iOS apps, ringtones, and books (PC only) that you've already downloaded in your iTunes Media folder. If you have an iOS app, ringtone, or book that's no longer available for redownload, you can move that content to your iOS device while plugged in to a computer with a USB cable.


That last sentence links to another article that provides more detail:


  If you have media on your computer that you can't redownload from the App Store or iTunes Store directly on your iOS device, such as custom ringtones or iOS apps, you can drag and drop the media from your computer to your iOS device using iTunes.
  
  
  Connect your device to your computer.
  Open iTunes. Make sure that you have the latest version.
  On your computer, find the item you want to add to your device. If you're looking for Apps, Tones, or Books (for Windows) that previously appeared in your iTunes library, you can find them in your iTunes media folder.
  Drag the item from your computer to your iOS device in the left sidebar of iTunes.