If the version you have is working well for you, stick with it. It doesn't expire. While there may be a prompt, there is no mandate to update or repurchase.
Updates are cost free for six-months from date of purchase. After that, newer versions can be test driven on a trial basis, to check out improvements or new features. Hang on to your purchased installer and registration info.
And there's a free version (unlicensed) with limited features.
NCH does offer a subscription, where the release stays relatively current. It's just 18 cents per day - paid quarterly... https://tinyurl.com/2nc3vswd