OnAir now comes with signed code and a notarized package to help you get started without having to build it.
The package was built on macOS Big Sur (x86; it's a MachO thin binary); it might work on older versions, let me know if it does or doesn't.
Should you ever want to uninstall it, you can do:
sudo rm /usr/local/bin/onair
sudo pkgutil --forget net.pcable.onair