[armadsen]

> 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'm a Cocoa developer and go out of my way to do this. You can find and download the last supported version of my app for each major OS X release here: https://help.aetherlog.com/faq/oldversions/ (linked from the main page on the website). I've also intentionally kept the license key scheme the same so a license purchased today will activate any version of the app, even very old ones.

I hear not-too-infrequently from users who have an old Mac that they want to use for my software, and it's nice to have a solution for them. The caveat of course is that some things are inevitably broken with regard to (third-party) web APIs that have changed. But I'm lucky that my app's core functionality isn't dependent on an internet connection at all.