Image Capture shows no camera or scanner connected unfortunately. It's worth noting that I know the scanner works because I downloaded a free trial of Vue Scan and it successfully scanned the image.
I noticed a past user had this advice. I tried replicating it myself but haven't had any luck.
I searched for the Scangear program, which is part of the Canoscan Toolbox package, and found that Scangear on its own is available for all MacOSs up to and including High Sierra. But on the Canon download page, there's also a note stating that ICA drivers must be installed in order for Scangear to work, and there are ICA drivers for High Sierra. So I installed just the driver, then installed the old version of Canoscan Toolbox, which supposedly only supports Lion, and bingo, it works like it always did on High Sierra.
This is where the driver lives on the Canon Canada support site:
https://canoncanada.custhelp.com/app/answers/answer_view/a_id/1011497