Subscription only makes sense if there’s an ongoing service, e.g. processing in the cloud, cloud data storage, etc.
Apps that don’t need to be subscription:
- Camera apps like Halide or Filmic Pro, wtf
- Any todo / habit apps, the ‘cloud’ part is usually iCloud / Google Drive
- Notetaking apps, e.g. GoodNotes, wtf
- Duolingo, mainly because the contents of some lessons are outdated (missing audio, etc).
I don’t know if it fits all the prereqs of a FOSS, but there’s mycroft and there’s also jasper
But I have no idea how advanced they are, or how good their 3rd party integrations are.