Earlier quoted context omitted.
Unfortunately I’m not expecting to support anything lower than Big Sur. I tried supporting Catalina, and having to support an OS that I don’t use daily was hard… Sorry about that. :-(
I really wish Apple provided an easy, official way to run older versions of macOS/OSX in a VM. It'd be incredibly useful for testing, and for ensuring reliability in old build toolchains (say, for a years-old product that needs a security patch). I mean, it already has a hypervisor built in. :-/ They don't even make it particularly easy to script an installation of an older version in a VM. There are 3rd-party tools…
Regarding Mojave... Yeah I still use that daily. Catalina is a mess, and Big Sur can't be (officially) installed on my mac. I could've tried to have a go at backporting the OP app, but Swift is too annoying of a language and I don't know much of it.