WOW it turns out it IS possible but the method is EXTREMELY bad: you have to “copy” your m4r (ringtone in AAC format) files from Finder and then “paste” them into your connected device’s “Tones” section in iTunes. Drag and drop *does not work*. https://support.apple.com/en-ca/HT207955
also iTunes no longer includes ringtones in its library, but if you had some in your library previously they’re just sitting, totally abandoned, in ~/Music/iTunes/iTunes Media/Tones