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Not sure about version requirement but you can find that Mac app is written in Swift: https://github.com/mattermost/focalboard/tree/main/mac and Windows application in C# uses WPF: https://github.com/mattermost/focalboard/tree/main/win-wpf

Linux app seems to be just a local server supposedly with web browser as an interface.

Both these apps contain a few source files and all they do seem to be rendering and handling a webview. The "real" app seems to be this: https://github.com/mattermost/focalboard/tree/main/webapp.

Yes, github repo also mentions Catalina (10.15) and XCode 12+.

So, Swift would explain that..