Upgrades don’t only extend for one version. You were entitled to the version 3 upgrade. Topaz does things differently than most software companies.
The license policy for Topaz apps (since 2019?) is unlimited updates (including bugfixes, feature enhancements, even major version upgrades such as you got) within one year of your purchase of the license. After that you can continue indefinitely to use the product in the version it was in when your license expired–you just don’t get any new available updates.
If you want to start updating again, you can pay a renewal fee that’s lower than the retail license fee, to get another year of updates. You can do that the day your license expires or you can wait years before resuming your subscription for another year of updates without paying a financial penalty for having let the license lapse.
Don’t think the app is still improving? Don’t think an upgrade is useful to you?. Don’t have hardware that can run the new version, or don’t use the app very often? Don’t buy an upgrade license. Just keep using the version you have.
I think this is the fairest win-win pricing model for both software company and users. They get more money when they make the apps better. Topaz has apps they no longer updating. They don’t offer license renewals for those, but of course, you can keep using them as long as they work for you.
Note that the terms of their license does give them the right to change this pricing and licensing model in the future, so it may not always work this way.