DiffBot does something like this. https://www.diffbot.com/

I was wondering if someone would mention RSS (or Atom).

What I'd like to see is a site that generates diffs from webpage changes, and makes them available as an RSS feed you can subscribe to.

Ideally this would be a free service, and I wouldn't mind if the RSS entries were prepended with adverts (or required you to click through to a page hosted by the service in order to see the diff, surrounded by adverts).

Was it Kimono Labs? I remember years ago that everyone was raving about them, so I decided to give it a try. It was simple, nice, and a pleasure to work with. Fortunately for them, they got acquired a couple of years ago.