There are apps like https://offpunk.net/ which are explicitly low tech (and even solar punk). But there are also apps, which just happen to be offline first (think PWA etc.). And things in between, like Syncthing. Some might be self hosted other maybe local or distributed applications.
What’s your favorite offline-first app or tool?
I do get the appeal of things needing to work without internet, but it seems very broad as a category. People use webapps for things that used to be local, like Office 365 or Figma, or even searching in Google to do arithmetic, so the calculator app is offline first.
On mobile I think a more reasonable example is offline maps, I use OSMand myself but recommend Organic Maps to less technical people.
About offpunk, all browsers used to have that. Firefox still has the “work offline” option in the file menu. In offline mode you can go to any webpage that you visited while online.