[TL;DR] To use old scanners with macOS, use AirSane.
I recently got an old but high-end CanoScan 8400f scanner. As well as digitalising paper documents I receive in the post, it can also scan my developed films. I might use that feature one day!
Anyway, this scanner is from 2004, those bad old days when proprietary drivers were still a thing. And guess what, Canon will not support drivers for more than 7 years!
Using it on Linux was no problem, just plug it in and the drivers are already there, in the monolithic kernel. On MacOS, however, no such luck. I stumbled across some old Apple forums where the only viable suggestion was to use VueScan.
VueScan is a wonderful project that will hopefully help a lot of people keep their scanners out of the landfill, but I wanted to see if the open source world had something to offer. And it has!
A project called SANE - Scanner Access Now Easy" provides a common interface for all these printers in the wild. For MacOS, there is an AirSane front-end that worked well for me.
It took me a while to find it out, and when I wanted to leave the trail for those who will run into the same problems, I found that I am not allowed to post on old Apple community threads. That's why I wrote this post. And that post was what finally got me to start publishing my notes.