I'm the author, and I agree, it's not really possible for Cocoa developers to continue supporting old versions of macOS. The projects on my page have the advantage of
only targeting old OS X releases, and not modern ones.
However, what really helps me is developers who document when support for an old OS was dropped, and continue to make older compatible versions of their software available for download and purchase. I have spent countless hours digging through the Internet Archive, doing a manual bisect to find the last compatible version of some app. Sometimes only to realize at the end that this version won't work with new license keys.
Worst of all are apps that have auto-update mechanisms I can't disable, which automatically replace my old working copy with a new version that crashes on launch. Please, don't do this!
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Apps designed to be cross-platform are a different story. I think it's more than a little annoying that Google Chrome doesn't support OS X 10.10 and older, when the set of changes needed to support back to 10.7 is really quite minuscule, relative to the size of the Chromium codebase: https://github.com/chromium/chromium/compare/main...blueboxd...
If a single developer working in his free time is able to maintain backwards compatibility, the full force of Google should be able to do it too. Not just for weird people like me who are strangely emotionally attached to old versions of OS X, but for people in Guatemala who literally can't afford to upgrade their hardware!