I see that MacOS 10.10 (Yosemite) is still listed as acceptable for running the latest release of Xojo, and presumably applications built with this release. However, I found this week that the latest version of the Xojo Feedback app, is nonfunctional in Yosemite. It may or may not open a window when it starts up, but even if it does, most of the controls are missing and those that are present, are non functional.
Problems with Yosemite aren't limited to Xojo. There are a number of applications that claim to run under Yosemite, but don't. LibreOffice 6 is a good example, and there are others.
I've always believed in the maxim "If it ain't broke, don't fix it," and so I've stayed with Yosemite as long as possible. I dread system upgrades because I usually spend at least a week getting all of my applications working again. And, since so many applications claim to run under Yosemite, there didn't seem to be a pressing need to change. However, this week, I finally gave in and updated to Sierra. The Xojo Feedback app is now working, along with a bunch of other things that never worked correctly (or at all) with Yosemite (even though they were supposed to).
So, just a heads up for anyone creating applications with Xojo: Don't assume that it's going to run under Yosemite (or other old versions) without actually testing to make sure that it really does work.