Seems like a feature of our fish is currently broken, this is at the first start, just after guix package -i fish (and no config files present, all sitting in backups):
> fish fish: Unknown command 'python -B /gnu/store/f80pivij8kfn1r49k2a3194kic3yhv7g-fish-2.4.0/share/fish/tools/create_manpage_completions.py --manpath --progress --cleanup-in ~/.config/fish/completions --cleanup-in ~/.config/fish/generated_completions' /gnu/store/f80pivij8kfn1r49k2a3194kic3yhv7g-fish-2.4.0/share/fish/functions/fish_update_completions.fish (line 2): python -B $__fish_datadir/tools/create_manpage_completions.py --manpath --progress --cleanup-in '~/.config/fish/completions' --cleanup-in '~/.config/fish/generated_completions' ^ in function 'fish_update_completions' called on standard input -- ♥Ⓐ ng0 -- https://www.inventati.org/patternsinthechaos/