@slaine It's a very close clone of NeXTSTEP 3. I've been building it for a while - mainly for myself. It is built on top of GNUstep, Backbone and some other userspace apps (some of which have been abandoned) as well as applications/framework written entirely by myself. My base system runs Slackware64, but I also ensure it runs on OpenBSD so unlike other projects (NEXTSPACE mainly) it doesn't link/require anything specific.

The WindowMaker version running underneath is very custom too. I try to submit patches and issues upstream to GNUstep projects where applicable (mainly crash fixes) but given that a lot of my interactions with people in the community get ignored, it's been hit or miss success wise so I stick to working alone. There's other like me too that work on their own environment (https://onflapp.github.io/gs-desktop/index.html) but as always, they have very different goals in mind.

My Mail app even supports various types of Mail-faces. Both local and online. It's built on GNUMail but again, quite a bit modified to facilitate all this.