Thanks for linking to that issue. Not 100% sure it’s the same issue, but seems like it. The author of the issue doesn’t state what he means by ‘broken’.
My issue is about an non-interactive/dumb shell that inserts literal tabs. There is a similar issue where auto-completion doesn’t work for any software not specified in mkShell. So in that case git would not auto-complete anyway, because its share directory is not listed in $XDG_DATA_DIRS.
I am not sure what the underlying problem is, if there is any real problem here. Is nix develop pure (or nix-shell)? You can access tools from the ‘parent’ environment, like git, which begs the question why git is in $PATH, but its share directory not in $XDG_DATA_DIRS.
Also related: bash autocompletion in nix shell (nix command) · Issue #4453 · NixOS/nix · GitHub