I admit I've not checked and have just assumed the package list is both ideologically- and mindshare-crippled, due to the GNU on the name and the seemingly low rate of use, respectively.
Nb I think free software goals are great and all but if I need Slack for work or need my proprietary wifi card to work using a binary blob then I f#cking need those things, and it's nice if my package manager can install them for me. It's also nice if it's got enough eyes on it that I essentially never run into a broken package, even when installing kinda-obscure things.
You can use Guix on distros with impure repos, not just the GNU Guix system, and use the host distro's packages for proprietary software (if you can't just use Slack's web client, or use a laptop with a freedom-respecting wifi card / a little dongle / ethernet).
(Edit: looking at https://guix.gnu.org/en/videos/everyday-use-of-gnu-guix,-par... from the top of https://guix.gnu.org/, basic installation and removal of packages looks exactly like apt, literally the same command line with 'apt' replaced with 'guix' and 'sudo' unnecessary because you can install packages per user.)