Longevity of "mac-only" app version?

As an Indy developer, I'm contemplating using Sketch, so the Mac-only purchase is an option. But I have concerns. Perhaps someone with experience can enlighten me...

Once the year of updates ends for the Mac-only licence, you're stuck with that version of the app forever. Which doesn't really bother me, because I'm not a professional designer, and only really need the app during the design phase, so I'm not needing the latest and greatest.

What bothers me is, how many version of macOS will the app be able to run on going forward, before it's essentially bricked, and you see the inevitable "This app does not run on this version of macOS" message?

Anyone care to share their experiences with this? Thanks.

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