I've purchased "Due" via the App Store, and it tells me I'm entitled to free feature upgrades for 1 year, shows a date when my unlock period expires, and offers a longer-term subscription. So it must be possible.
Yes. They seem to gate on a feature-by-feature basis, know when you bought the license, and you don't get access to the new features after 1 year (unless you're on their subscription plan). You do continue to get the latest app, which includes bug fixes, but the features released past expiration aren't available to you. https://dueapp.zendesk.com/hc/en-us/articles/360053244591-Wh...
That is so much more work than simply not getting the latest update. Imagine if sth like IntelliJ put everything behind feature gates instead of just telling you to download an older version…
I believe Reeder is doing kind of this feature. They release every year or every other year a new version with new version name like Reeder 5. So, if you want an upgrade, you just purchase Reeder 5, but you can always download Reeder 4 if you have purchased it before.