iTunes 12.7 not supporting custom ringtones

You can still use custom ringtones. The following may help:

Use tones and ringtones on your iPhone, iPad, or iPod touch - Apple Support

and

Create and manage ringtones in iTunes

If you want to give feedback to Apple regarding changing iTunes back to how it used to handle ringtones: Feedback - iTunes - Apple

You no longer use Sync but you can manually move custom ringtones from iTunes to an iPhone. Read the "Add ringtone to device, iTunes 12.7 or later" section of Create and manage ringtones in iTunes

and/or the "Move ringtones from iTunes to your iPhone ..." section of Use tones and ringtones on your iPhone, iPad, or iPod touch - Apple Support

I have just successfully created 3 x custom ringtones for my new X ,it may not be a simple process ( anything but) however it does work with a limit of 30 secs per tone

Follow the links FoxFifth has provided it will ultimately work but patience is required

Hope you can sort it out it took me hours 😟, detailed study of the instructions are imperative

but very satisfying when working 🙂