I wrote to the maintainer in late 2018 and he informed me that the project is no longer under development. He mentioned that CoreObject (coreobject.org) would still be active, but that was over two years ago and I can't say for sure.
That's unfortunate about Étoilé, which looked promising and cool. Last summer I wrote an article about the current state of GNUstep, and I was hoping that someone would pick up from where Étoilé left off.
However, I just saw a recent comment from one of the current GNUstep maintainers that said that the GNUstep Foundation Kit is now at par with macOS Catalina, and work will begin on updating GNUstep's Appkit implementation, which is currently roughly compatible with Mac OS X Tiger.
Making GNUstep more compatible with recent versions of macOS's Cocoa API will definitely help with enticing more developers to the platform.
