From Ulysses’ FAQ:
What happens after my subscription or trial ends? Can I still access my texts?
Definitely. Ulysses is in read-only mode, meaning you can still access all your sheets and export them using any export format.
I strongly believe that to alleviate concerns over “renting software” instead of owning a copy, Ulysses (in this example) should still be fully functional when a user ceases paying their subscription, but it would stop receiving updates and new features. If a developer was feeling extra generous, they could support new OS versions and security updates.