It could be useful to know which packages I use that are outdated.
That needs some further explanation of what you consider “installed”.
Are you talking about…
- /nix/store paths (including older generations)?
- Packages in environment.systemPackages?
- Packages referenced somewhere in /run/current-system?
- Packages installed through “nix-env -i”?
Also, what do you consider “outdated”?
- Newer version available in your current channel
- Newer version available in the current release-XX.XX channel
- Newer version available in nixpkgs-unstable
- Any distro (e.g. Arch) has a newer version
- Unsupported package version by upstream
There’s a bot that tries to keep nixpkgs up to date (see for example the pull request 79332). Depending on your needs that could be a good starting point.