It does not work and we don't plan on maintaining these anymore.

The flake files are being moved to
nix-community/neovim-nightly-overlay#483
instead.

There seems to be a nice update possible nix-community/neovim-nightly-overlay#483 (comment) .
I will try to do the whole update/annoucements today and will merge this afterwards if that's ok with you. Having the flake disappear without proper announcements will annoy several people.

flake moved to nix-community in nix-community/neovim-nightly-overlay#516 .
Announced on reddit via crosspost on r/neovim and r/nixos at https://www.reddit.com/r/NixOS/comments/1cz67bm/flakenix_moved_from_githubneovimneovim_to/ .
And on nixos discourse at https://discourse.nixos.org/t/neovims-flake-nix-moved-to-nix-community/45887 .

Thanks to the core maintainers for accepting the flake to be committed in the first place, then merging nix PRs, and finally let nix users handle a graceful migration.
As development sped up, it became a hindrance to maintain so let's have neovim maintainers focus on neovim instead and the nix community build a richer neovim flake.

There are some orphaned references left in BUILD.md referring to this flake that's been removed.

@farnoy wanna make a PR?