I think that at least for now, it would be better to link to https://github.com/NixOS/nixops, as the old link just links to an obscure in-development HTML document, and the github link starts with a warning "NixOps is in low-maintenance mode and probably not suited for new projects. Use at your own risks", and also links to an experimental NixOps4.
The link to NixOps was corrupted by mistake in this PR: https://github.com/NixOS/nix-pills/pull/181/files#diff-a53cbc0f08ac225fd5073d54147a9a6ba2ebbdbd0e5164711ab362d0439cc27aR265
It was changed by mistake from http://nixos.org/nixops/manual/#chap-introduction to https://nixos.org/manual/nix/stable/introduction.html.
I think that at least for now, it would be better to link to https://github.com/NixOS/nixops, as the old link just links to an obscure in-development HTML document, and the github link starts with a warning "NixOps is in low-maintenance mode and probably not suited for new projects. Use at your own risks", and also links to an experimental NixOps4.