Nixpkg does quite a good job in tracking those issues, imho. https://github.com/NixOS/nixpkgs/issues?q=is%3Aopen+is%3Aiss... is a list of security issues. Most of them generated by automated scans of nixpkgs-unstable.
But as far as I am aware, there's no mailing list or so for receiving notifications upon critical vulnerabilities(?). https://nixos.org/community/teams/security.html mentions github issues, discourse and matrix. Triaging security issues requires significant work and it's a task even more traditional distros like Debian often struggle with.
One thing I'd like to see eventually is an option to nixos-rebuild and other to emit warnings if installed packages are affected by known vulnerabilities. I think that should be doable and would maybe raise awareness and provide most visibility to the issues affecting most users.
https://github.com/flyingcircusio/vulnix does something like this, but it's currently a third-party tool