We generally announce the system requirements for a product when we announce the date it will be available for sale, but our general approach is to support the current shipping operating system (i.e. the version you’ll get when you buy new hardware) and the previous version. When OmniFocus 3 ships, the current operating system will be Mojave—so we’re supporting Mojave and High Sierra (and with Mojave such a moving target right now, staying on top of evolving compatibility issues is keeping our test team very busy!).
This is partially driven by Apple’s development tools: for example, Xcode 10 adds support for theme-sensitive color palettes in order to support Dark Mode, and High Sierra includes support for looking up colors in those palettes (even though it wasn’t part of High Sierra’s public API). But Sierra does not—so in order to support Dark Mode on Mojave (its headline feature), you have to write code which will only run on High Sierra or later. (You can, of course, build a totally different implementation to maintain compatibility with earlier operating systems. But now you’re not getting the benefit of adopting the new tools Apple has provided—it’s just an additional cost, slowing you down—and the independent implementations are going to have very different bugs.)
