We do our best to make sure our apps will continue to work normally after you upgrade your operating system. OmniGraffle 5, for instance, worked across ten years of macOS upgrades.
But we can’t guarantee this, because we can’t know what incompatible changes will come in the future.
There are two important things to know:
If you don’t upgrade your operating system, then the versions of the apps you currently have will continue to work.
If you do upgrade your operating system, you should expect to update your Omni apps to their latest versions. Often this won’t be a paid upgrade — but if we released a major new version between the OS release you were running and the new one, then it might be.
As part of our development and testing work, we run beta versions of upcoming operating systems, and when we see something break in one of our older apps, we often file a bug with Apple. (It’s not always straightforward enough.)
We also create a page on our support site that lists the versions of the apps that have been updated for the latest operating system, and it lists known compatibility issues with older apps.
At this writing we haven’t done pages for iOS 12 and Mojave, since they’re still in beta and things could change between now and when they ship. But you can see, for instance, our page for High Sierra.
(And: for businesses we offer a maintenance contract, which provides updates of our apps year-after-year. If that interests you, please contact email@example.com.)