Android, Windows, macOS, Linux, any platform but iOS, really. There's little stopping you from taking the Java library and stuffing it into a desktop application for a single everything-but-iOS notification service.

I'm not sure where the restriction lies, but with AltStore being easily accessible and Apple being rumoured to allow users to install their own apps like normal, it's quite possible that we will have an iOS version soon. Of course nobody will work on it until Apple fixes their restrictions, but I wouldn't be so pessimistic as the authors of the website.

"Fix" is a strong word. I was of the opinion that both Google and Apple have limited background services heaps of the years for the purpose of saving user battery life. Why should this one be an exception?