Github provides a nice UI that allows to check the work done on the NixOS community fork of the installer. According to it, the only DetSys commit to the repo was a README fix by Graham. Maybe they helped make sense of the existing code - but it doesn’t look like DetSys put much effort into upstreaming the installer.
Not to mention the fact that there’s no need to do that to begin with. DetSys installer is already on the front page of nixos.org, it is the more known installer, and it supports both CppNix installation and DetSys Nix installation. There’s not much to actually fork and change - just changing the branding and removing DetSys Nix code paths, mostly.
For disclosure, the reason I know that is in the past few weeks, I ported and authored a bunch of patches to lix-installer, the Lix flavor of the installer. I haven’t reached out to anyone from DetSys to help me with this. I asked other Lix community members one or two questions about it, but otherwise my work was fully solo (aside from porting upstream PRs, where it’s not me who made the work to begin with). The project isn’t terribly complex: more overengineered and riddled with tech debt, if anything. It’s not too hard to fork and develop mostly- or entirely self-sufficiently. To clarify my role: I don’t hold any positions of authority or any formal roles in the Lix project, I’m just a random guy who contributes to it.
But once again, there’s no reason to do that aside from branding - DetSys installer already supports installing CppNix. Lix is a separate project that installs a separate daemon, so it explicitly cares about getting the branding right, and it also needs to have self-sustainable installer because Lix has already diverged quite a bit from CppNix, and will only grow farther. Those concerns don’t seem to apply to NixOS community at large, so shrug